Coolant boiling over from reservoir.

[Seany_boy]

So I just bought a 2002 impreza wrx wagon with an ej205 in it. When I bought it it the reservoir was completely full to the top of the jug with coolant which i was a little suspicious about but otherwise it ran fine with no issue. Now the cars a manual and I don't know how to drive manual yet so when I tried to start it at home, I didn't realise it was in first and only realise after tried to start it the second time. I heard something boiling at that point but left it cause it was near midnight and I had work the next morning. Found out it dumped half its coolant and did a coolant flush. Ran for about 8 minutes then it dumped its coolant at idle. After some research I assumed the thermostat was the issue and after I replaced it, it still lasted about the same time before it started to boil while driving. The radiator cap is fine, the hoses are fine from what I tested and while there is what looks like old milky stuff in the thermostat housing, but the oil is fine and when I drained the coolant the first time it was fine besides having some stuff in it, so I don't think it's a head gasket but I could be wrong. Help please

 

[Tonal443]

Friend get that car to a machnic uve got a serious problem in the engine