Thermostat replacement is way easy, they are on the top of the motor at the front, and they cost about $8.00, plus the coolant that you will need to replace when you change it out, unless you want to drain your coolant system before you get started. Sounds like it may not be opening when it should, or that you need a system flush, so kill two birds with one stone and do both at the same time. Just remove the upper heater hose from the motor after draining your coolant, then remove the hose fitting that the hose attached to, the thermostat is underneath the fitting, or will be attached to the bottom of it. Make sure you get a good seal when you put the new one on.