1986 cabrio gti rough idle problem

Hey folks,

Looking for some advice again, the problem I have is my car is sounding very rough when ideling it starts pretty much every time and runs not bad for a couple of minutes then starts to struggle, when I hit the accelerator it sounds as if it's missing then picks up when I hit higher revs it's pretty much been like this since I bought the car I tryed turning the idle screw but doesn't seem to cure it, could it be the timing or the mixture Is out? Not that clued up on these things, for information I have changed out the spark plugs,ht leads, rotor arm and distributor cap I also replaced a ruptured rubber valve housing but none of this seems to have had much effect on the running of the engine any advice on what it might be or any tests I could carry out would be much appreciated.

Thanks in advance.