I agree, thermostat or hoses. To check the thermostat just put it on top of the stove heating element and see if it opens up or not. Even this may be deceptive as I have had a couple in my day that would stick, then open, then stick. Once had a 62 impala 327 that would do that and I could tap on the t-stat housing and it would open. A good way to check the hoses is when it get hot some hoses that appear to be ok will actually "suck together" as the heat rises. Just adding more causes and effect to the same situation!!
