Water Heaters - high limit switch problem

I'm looking for some information about a high limit switch that keeps on tripping. I installed an 80 gallon whirlpool dual element heater about 6 months ago and up till a week ago I've had no problems. I have a solar heater that runs during the day but shuts down at night. As of recently the high limit switch keeps tripping. The thermostats are both set at 125. A couple of days ago I couldn't get the switch to reset so I replaced the high limit switch and upper thermostat assembly. I could get the new one to reset, However, It also tripped a day later. The problem that baffles me is that it trips at night when the solar panel is off. :wall:

I'm not real familiar with your setup...........take pictures of your situation......especially the high limit switch.