Tony it sounds like your engine would benefit from a session on a rolling road with a good engine tuner.
Unfortunately a mobile tuner will only be able to set your idle mixture correctly which is obviously not where your problem lies.
on a rolling road they can check the jetting at other driving conditions and change it to suit.
If the black deposits are wet and oily it would suggest that your engine is showing it's age.
If it's black and dryish its probably just mixture related.
Do you have a local rolling road?
Tinka had his done recently and it made a real big improvement.
http://animatedgif.net/flags/-flags-uncat/Wales.gif
Kev Davies
Dax, 383ci Chevy Stroker, Tremec TKO
South Wales
:tu :tu :tu