How many times has the cartridge been refilled? Has it ever run dry, or almost dry?
I refill 5 times, by then the cartridge is not going to be reliable. The ink serves as a cooling agent, if it gets too low the feed mechanism will overheat. You can also get an airlock, that will usually correct itself if the cartridge sits for several hours.
More than likely you need to start with new cartridges, and keep track of how many times you refill.
The best ink, and the best prices I've found,come from Stratitec.
www.easyinkjetrefill.com
This is it's first refill. I've never had any luck with any brand over 2 refills.