I just replaced the head on a 1996 850 glt 2.4 liter for the second time with the same issue. It started and ran great. Then after about 5 minutes pressure is pushing the water back thru the water resevoir. My only conclusion at this point is an exhaust valve stuck possibly. The car has power and runs good but this issue is haunting me. Can you help or give me a suggestion.

 

Time to do a leak down test

 

What does "pushing water back through the reservoir" indicate ??

Is the coolant actually being pushed out of the reservoir with the cap on?
It is bubbling inside the tank excessively?
Does the vehicle overheat?

A leak down test would be good as would a test at the reservoir for hydrocarbons. If you do have exhaust gas entering the cooling system and this is the second time you've done the gasket there is a chance you have a cracked head or block.
Autozone loans out the hydrocarbon tester, you have to buy the fluid.

 

could be leaking head gasket, could be over filled, could have air in the system, could be stuck thermostat, and a number of other things. Think simple. The science of physics can explain almost everything.