pressure building under oil cap

I have a 92 volvo 960 . I recently noticed a lot of oil on top of the engine. I finaly found that if the engine is put under a load (enough to spine the wheels in gravel) then it builds quite a bit of pressure under the oil cap and oil starts comming out from under the cap.
The pressure will hold for a while. I can slowly remove the oil cap and it is still there. It compares to the pressure under the radator cap when it is warm.

Any ideas? Am I correct that an inline 6 dosent have a pcv valve? Could this damage the valves or seals?

Is this (http://www.brickboard.com/FAQ/700-900/EngineSealsBeltsVent.htm#960FlameTrapinB6300Engine s) your engine?

my old 240 and my 740 use what is called a flame trap which acts like a PCV. Most likely it is clogged. On my 740 it's located under the intake manifold . Try this URL www.volvoclub.org.uk/faq/FAQSummary1.html