GMC Repair: 1999 Envoy - Misfire, gmc jimmy, carbon buildup

Question
I own a 99 GMC Jimmy Envoy which has the 4.3 engine.  It has had a problem with misfires which turns on the Service Engine Soon light.  The light usually goes off within 1 to 24 hours.  This vehicle has had this problem for a least a year but it is getting worse.  We have within the last year put on all new parts like sensors, plugs, wires, fuel pump, catalytic converter, transmission.... The fuel pressure is good, vacuum is good, good compression...  The engine generally runs rough (misses) with cold starts, and it noticeable at highway speeds also.  We have taken the vehicle to a shop and had the GM mechanics check it out and they were unable to find the problem.  Do you have any suggestions?

Answer
could be a few things but i own a 1999 and these are the issues i have had, 1 i had a plugged fuel filter, i notice you had the pump changed what about the filter?  2 i have a misfire on #1 cylinder and it is carbon buildup on the exhaust valve which requires removing the head and having it sent out to reconditioned.