T100 wont run

[Mark N]

My 97 T100 SR5 3.4 4x4 5sp Ext Cab runs perfectly when there's a mechanic nearby, but otherwise it either wont start (turns over but doesn't fire) or it runs on seemingly only part of one cylinder with lots of sputtering and coughing and ugly black gassy exhaust smoke. On the rare occasion when it runs, Diagnostic shows no problem. I've been changing and checking components one by one, but so far have not found the solution.

Any suggestions??