My state select water heater is geothermal/electric. Few days ago water got scalding hot then went cold. Next day it was fine. It's done the same thing twice since. Now I get more tepid water occasionally. I've tried the reset several times and it works temporarily.
Any ideas?

It sounds like a control sticking closed. This is something that should be fixed right away before it causes major damage. If you can send me a wiring diagram I can see what may be the problem, but you will have to use a meter to test it because you cannot guess on electrical devices; you have to be sure before you change a part that the part is bad. Usually, these devices have three thermostats: one safety and two more for the top and bottom. Find the manual, scan it and send it to my email account and I will take a look at it.