This is very strange indeed. How long does it take to go up 3 degrees
and what is the ambient temp? The only possibility left would be a
short that is somehow heating a metal part of the water circulating
system. This seems very unlikely, there should be a ground fault
interupter on the supply circuit and a short to ground would trip it.
I'd get an amp meter and measure how much current it draws with
nothing running, then with the pump, etc. to see if there is a higher
than expected current draw. You could also turn off the AC, then
feel around all the components with your hand to see if anything is
hotter than it should be.