So, changing the cap was not a succes... and you say you have a lot of pressure caming when no cap on... So, you have a lot of pressure in the crankcase. Which is normal for high milleage car. This is where your flametrap (PCV valve) need to work better than ever; the pvc valve need not to be clogged because it is where all the crankcase over pressure go to; if it's clogged, the pressure will have to go somewhere else, it will find a way... oil cap is one of those way... and sometimes, is can cause engine seal to blow because of high pressure in the crankcase. The pcv valve (flametrap) is located beside the throttle body. It's a plastic elbow attached to the main throttle body elbow (between the air filter housing and throtle body housing.

Hope this help, if you need more info, just ask: bruno_champagne@yahoo.ca

Bruno