Oldsmobile Achieva 1997 - Thermostat housing

I got my Oldsmobile Achieva 1997 4 cylinder december last year. In January this year, the thermostat housing broke for the first time. Today it broke again, but now for the FOURTH time, after I have taken to the mechanic every time it broke before. The mechanic suggested me to change motor mounting and radiator hose, which I did. I do not think he knows exactly what the problem of the car is, because, as you can see, the problem persists after I have spent about $500 on it. I wondered if the problem was in the way I drive it, as I am not used to auto transmission cars (I am Brazilian and most of cars there are manual transmission), but I couldn't figure out anything.

The car doesn't worth more than $1500, I believe, and if I hadn't earn this car as a gift, I wouldn't spend more money on it.

Does anybody have any idea of what can be happening to my car? I would appreciate your help!!!