Audi 90 2.3litre 20V misfire

[MikeT]

Hello all. First post. Bought my Audi 90 about 12 years ago and a very intermittent (almost non-existent)  fault has become full time. Initially a very slight and very infrequent misfire has morphed into not running at all on cylinder 3. I have done all the obvious things such as fitting new plugs, leads and distributor cap and I have ruled out (by substitution) the fuel injector on cylinder 3 and the ECU. Compression is good on all cylinders and with plug 3 out of the block it still sparks well, but is completely black (dry, not oily). Is the fault likely to be in the distributor body itself and where can I get the part number? (not easily visible in situ). The engine is 2309cc, and the car was first registered on the 1st Jan 1992 (J5 AUB). Thanks in anticipation.