Overheating. Temp drops after restart?
 

So, I got my 3kgt about 3 months ago as an "if you can fix it, you can have it" from my mother. When I got it, it would overheat immediately after starting it. I checked the upper and lower radiator hoses and concluded it was a bad thermostat, as the lower hose was cold, upper hose was hot. After changing the thermostat, I am able to drive it for about 25 minutes, and then out of nowhere, the temp shoots up again. However once this happens, all i have to do is pull over, shut the car off and turn it back on right after, and the temp drops as if nothing ever happened. This being within 20 seconds of it overheating. Any insight on this? I was thinking it could be the water pump, but lord knows I dont want to go through all that extra nonsense and it end up not even being the problem. ( I've changed waterpump/timing belts in the past. They suck) oh, also, when I pulled over and popped the hood after the temp went down, i noticed the reservoir had overflowed and sprayed coolant everywhere. (Res cap is bad so it pops right off) But any advise is appreciated. thanks in advance. :)