Ocassional Starting Problem

[chickinwire]

I have a 1991 VZ Legacy, about once or twice a month, when starting the engine, there will be what sounds like a big air discharge from the engine. The car will still turn over and after a while of just constant turning it over it will start, but will only run on 3 cylinders for a while, runs horribly, struggles to idle, after a couple of minutes of driving, its fine and everything is sweet.
Has anyone had/got/now of similiar problems.
I thought it may have something to do with one of the pistons stop half way over the air inlet or something causing problems, but am really unsure.
Any help or ideas would be great cheers