1989 S3 Spider injector problem?

I think I may have an injector problem with my Spider. The car is not quite running on all four cylinders and I have had a plug foul up on one of the cylinders a couple of times, so I'm wondering if I am getting a slight leak from one of the injectors. When running, the car sounds rich, as if the choke is on, but it does start ok.

I should point out that the cylinder head has just been overhauled with new valve guides and valve seat inserts. However, the above problem was apparent before the rebuild, so I've cured the smokey exhaust and the rattle from the loose valve seat, but not the actual running!

Any suggestions welcome. New injectors will be expensive, so I'd like to be sure before proceeding. Are the injectors common to any other car?