BMW: BMW 325 M50 engine, oil filler cap, coolant system

Question
Hi Josh. I have a problem with my 1992  engine. It keeps pressurizing when the thermostat opens. I have replaced the stat,water pump,and the rad is very good. I have had the head off and there is no sign of gasket damage. I have had the head skimmed and pressure tested which they say is O.K.
There is a slight trace of oil in the water with white sludge on the oil filler cap. It starts and runs fine but again pressurizes when the stat opens. Any further suggestions that you might have would be appreciated.  
best regards ROY

Answer
Roy,
 If the system is pressurizing, then it's because steam is building inside the system (air) or cylinder pressure is excaping through the headgasket into the oil/coolant gallery's.  The Headgasket may have looked fine, but there is still a possibility for a pin-hole somewhere that's allowing the pressure from the cylinders to get into the coolant system and pressurize it.
 Good luck,
 Josh