help required golf gti 8v k-jet

hi if anyone can help it would be very much appreciated i have got a mk1 golf gti 8v k-jet car is running slightly rough i have had timing set it is spot on changed the usual thing new plugs,leads dizzy coil pack but still the same but when car has been running for a while i took oil cap off top of engine and alot of air come out has anyone had this before or does anyone know what the cause could be got told it could be head gasket or could need new piston rings can anyone help thanks craig

it may be your fuel system there are lots of checks to see and some are listed down the side in the technical box, if you feel confident to tackle them yourself.

i'd guess that the engine isn;t breathing right,    check the pipes are clear coming from the rocker cover to the air filter box, also read into old  posts about oil pressure……..