weird idling problems

[Guest tkch55]

my friend's stock R33 gtst with just a catback exhaust is currently experiencing weird idle

cold idles really rough, though stays at a stable 1200, but if you pull off the oil cap or dipstick, then it starts jumping up and down

when the engine is warmed up, idling sounds more normal, still gets unstable if oil cap's taken off, though not as bad as when it's cold

another symptom is if you rev the car in neutral, while the revs go back to idle, you can hear a metallic rattling noise. this happens regardless of whether the engine's warmed up or not

from what i gather, the weird idle with oil cap off sounds like it's loosing compression?

the car was rarely thrashed, it was started two days ago and it was running fine

oil change was done at approx 5500kms ago, atm, the car was recently involved in an accident when a car ran the redlight and hit him from the side, resulting bent drivers side rear quarter and driver's door (this is another story, will post it in NSW section later), tho irrelevent but car's currently not very drivable so we can't get it to a mechanic's place to sus it out in a short time, hence the post

car gets started every 2 days or so, didn't do this in the last few times but it started today

thanks for all your help in advance

cheers