(This is deaconjim) You may have the lower element burned out. The top element will heat some of the water, and it seems fine until you start using enough to get to the water at the bottom of the tank. If it is the bottom element, you will have plenty of hot water, but it will take a long time to recover after each use.
I don't know. The same thing happening this morning. My husband up over an hr ago and things fine. I just took a shower with just the hot water. At first it was room temp and then cold. Do you have to replace the entire heater or can you just change elements if that is what it is?