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A few possibilities come to mind.

I believe there will be an internal sensor that will shut it off if the pressure drop across a filter gets too high. If it is on the edge of shutting off, it might be because the filter is just too dirty but it might also mean that the sensor is flaky. I just found your model at this link and I see that this unit uses a HEPA filter… these are already much higher pressure drop than other types of filters. This by the way is the problem with all HEPA filter units….. there is often no way of knowing when the filters need to be replaced. There will be a pre-filter that is washable but the as for the HEPA filters, they can be cleaned by vacuuming them out but eventually they get to the point where the filter becomes internally clogged so that there is no alternative but to replace them. It’s a pet peeve of mine… the units should give an actual pressure drop reading as opposed to just some lights that give a vague idea as to what is going on with the filters. https://alen.com/collections/breathesmart-fit50 By the way, if you do need to change a filter, these lights likely need to be reset. Could there be something a bit flaky with this ‘reset method’? Having said this, this might be part of an explanation of why it shuts off but probably doesn’t adequately explain why it comes back on when plugging it into another outlet. However, it might be part of the explanation since it represents a control strategy that is purposed to shut the unit off…. Have you tried running the unit with no filters at all? Does it still do this unexpected shutdown thing when run without filters?

By the way, I see in the operating manual that your unit has an ‘ozone safe ionizer’ but it can be turned off and on. Does switching the ionizer on or off affect whether the unit runs or not? It needs to be kept as a possibility that there might be something in the controls for this part of the unit that are acting up. https://www.manualslib.com/manual/780439/Alen-Breathesmart-Fit50.html?page=5#manual

Along those same lines, the manual says that your unit has a feature where it in “Automatic Mode will use the SmartSen-sor Technology to adjust the speed of the unit depending on the amount of particles present in the air.” When it shuts down, is the unit set in automatic or manual speed mode? In automatic mode, there obviously are a bunch more sensors and controls that might have gone flaky…. By the way, on page 9 of the manual, it gives some specific instructions about cleaning the dust sensor that allow this variable speed fan to adjust to the desired speed. [How To Clean The SmartSensor - The SmartSensor uses an infrared light to detect airborne particles. After prolonged periods of use, dust can accumulate on the glass lens and affect the sensitivity of the sensor.] Have you done that?

Another possibility is that the motor might be running hot... there are quite a few reasons for that but depending on the design of your unit, the motor might have accumulated a buildup of dust over the years (even though it is located after the filter where the air should be clean) and now it can’t dissipate the heat and the internal thermal protector is shutting it off. You might want to look at that. I see that your unit has four speeds. Does it behave in the way you’ve described on all speeds?

As a related issue to the filter clogging up and the possibility that the motor is not getting adequate cooling, what is going on may relate directly to the type of fan that is used in this unit. These two types of fans that can get used and they behave differently with regards to power when the flow is reduced due to filter clogging. Specifically, the power will go down if it has a centrifugal fan and the flow was blocked off… it goes up if it has an axial fan and the flow is blocked off. I’m fairly certain that your unit uses an axial fan so a clogged filter will increase the power and make it run hotter…. You obviously can’t change the fan but knowing what type it is might at least tell you whether it is the type that is sensitive to taking more power and hence an increased level of heating that might shut the unit down. Reduced airflow has the secondary impact of heating it up even more since there is less air to cool it… Is there a build up of dust on the fan blades that is reducing the airflow? That obviously will contribute to the problem as well….

Lastly, are your units subject to voltage fluctuations? If it is, this might affect the speed of the fan and its power requirements (remember that power increases as the cube of speed so even a small difference in fan speed can have significant impact on the power). Do you live in an area of lots of wind turbines that might be coming on and off and affecting your voltage? Without getting into the details, it might just be the case that your unit is sensitive to voltage. These are really handy for measuring and to some extent, troubleshooting…. https://www.amazon.ca/P3-P3IP4400-Electricity-Usage-Monitor/dp/B00009MDBU?th=1 Plug this into your outlet and then plug the air purifier into it…. You can use this to monitor the voltage and amps and see what happens when you play with things like cleaning the filters, running it on different speeds, running it without any filters at all etc. to see what difference it makes.

By the way, on page 10 of the manual is this note… “I have an issue not listed in this manual. Please visit www.AlenCorp.com and send us a message or call 1-800-630-2396 during busi-ness hours. A customer service representative will respond to your inquiry in a timely manner.” Have you done that? You should only consider that as a last result because there is lots of other troubleshooting and things you can first.... and who needs to be hold for hours and then up talking to someone with an accent that can’t be understood. On the other hand, if there is something that is inherently flaky with your unit, you mind find someone honest enough to tell you that.