Is there any way of testing the thermostat beyond taking it out?
Another problem that can happen is that the thermostat can get stuck in the almost closed position. As a result it takes longer to heat up since it's not fully closed, and later can overheat since it's not fully open.
Haven't had coolant problems with my brick, but here are some checks with cars, some of which is relevant. If the thermostat is good it should be close to normal operating temperature after about five minutes or five miles.if say it takes half an hour, that's not looking good ( or with a car half an hour before the heater starts throwing out some heat) the coolant shouldn't be seen flowing until the thermostat opens, the hose shouldn't get gradually warmer but after the thermostat opens, the hose should be harder to squeeze after the thermostat opens.
If you have a temperature gauge the needle should gradually rise. After let's say five minutes, if the needle drops down just a little, thats the thermostat opening.