car wont stay runnin

[bimmer24]

Hey guys im new to this forum, and i need some help. I have a 98 328is and im having trouble keeping it running wen cold. It has 200, 000 mi and up until recently has ran pretty good. Once in a while if it sits for a few days, when i go to start it up it seems like it runs on a 4 or 5 cylinders, idling rough...after a few seconds to a minute of revving it, it goes back to a normal idle. But now every time i wanna start it, it wont staying running at all unless i rev it for more like 2 or 3 min... and once its warm it still has a little rough of an idle....If i pull the oil dipstick it actually idles better. I tried checking the vacuum at the oil separator and there was sum vacuum..but if i unplug it, it dies.. But i dont get y it idles better w a me pulling the oil filler cap or the dipstick. Im sure its something simple that i cant think of, but would appreciate some help if possible, thanks.