Car won't shift into overdrive (when first starting out), running poorly, stalls out, trouble starting again, sputters & sounds like not all the cylenders are firing, black smoke that smells like gas. What should I be looking for?

Optional Information:
1997 Pontiac Sunfire 4 cyl

Already Tried:
Taken it to a mechanic 3x's to no avail! "Knows it running poorly, but don't know what the problem is" -its happening more frequently! Help!