When you rebuilt the engine did you have the head rebuilt or did you just resurface it. Chances are you valve seals are shot (explains the sooty spark plugs). Check all four plugs daily. If new plugs are fouling, your valve seals are gone. Happened to me <img src="/forums/images/graemlins/frown.gif" alt="" /> Also, when you built the engine, did you put a new oil pump in it? Your timing chain tensioner is oil pressure driven. Bad oil pressure at the tensioner will cause some timing issues. Happen to me <img src="/forums/images/graemlins/frown.gif" alt="" />
I got mad and tore it all down, new short block, rebuilt the head with new valves, guides and seals, also changes clutch while in there.
Watch those plugs and LISTEN to your timing chain.