A lot of posts mention faulty thermostats as a potential cause of cooling problems and that news ones can also be faulty. Here's an easy check:
Before installing or re-installing an existing thermostat, boil some water in a big pot or tea pot. Get a Pirex measuring cup (preferably the 2 cup size) and cooking thermometer. Put in about 1/2 to 2/3 cup normal tap (30 to 40 C from the faucet) water and then add boiling water until the temp is 80 C. Use the cooking thermometer to gauge the water temp.
Toss in the thermostat and see what happens (make sure it is fully covered by the water). Slowly raise the water temp by adding more boiling water until the temp reaches as close to boiling as possible. To reach boiling you may need to put the thermo in pot on the stove. Rest it on a big plastic spoon/spatula so it doesn't make direct contact with the pot.
Aside from testing the thermo, you can see how open it really is at various temps.
Not fancy, but it works!