Thermostat? or Head gasket?
 

after a around 30 mins of driving, my 95 wrx's heater starts blowing cold... then the engine temp starts creeping up, it used to come back down after 10 mins, but i have to stop now and let it cool. when i have stopped i noticed water coming through the over flow.

I immediatly thouhgt combustion gases were getting into the cooling sytem but am hoping its just the stat sticking. cant see any water on the dip stick, and oil filler is free of white sludge.

anybody have an opinion?

driving ok? any white smoke?

Is the bottom hose hot or cold? Could be stat or even water pump, fingers crossed its an easy fix:thumb:

Make sure your rad/pipes arent freezing

yea sounds poss like water pump but id sniff test it first befor you strt pulling bits off mate

Firstly top up and bleed your cooling system to make sure there is no air in it (make sure you have the interior heater temp dial turned to hot)

Now run the car and get it up to operating temperature. Check both the top and bottom hoses and the rad get hot which will indicate that the thermostat is opening. (easier to do with an infared temp probe but when you haven't got one you have to make do)

Check to make sure the cooling fan operates at roughly the correct temperature by use of the dash mounted guage.

To check for hydrocarbons in the coolant you're going to need a test kit. You can still have a headgasket failure without the water and oil mixing as it will just have gone between the combustion chamber and the water jackets.
You can also check for the system over pressurising if you have a coolant pressure tester available (water pump operation can be checked also when using one of these but you can do it by leaving the header tank cap off reving the engine and checking the flow into the header tank)

One easy way to get it checked is join either the AA or RAC (online offers are around £20 to £30 and even cheaper when you use cashback sites such as quidco etc) and then ring in telling them it's overheating. They should do all the checks such as pressure test, hydrocarbons, thermostat operation etc.

Lots of bubbling through overflow = Headgasket or worse 99%. Do the sniff test.