[tblake]

the early 4.3 vortech's had problems with the CPI injector/poppet valves. I've seen some that would leak externally, causing 3 cylinders on one bank to run rich and the other 3 to run lean. I remember being able on some of them to remove the throttle body and have a peek inside to look for washing. (this was on later 4.3's. I think yours this may not work). Also 4.3 vortechs have slightly higher fuel pressure in order to open the poppet injector nozzles. If the pump is weak, it will not supply enough pressure to open the valves.

I'm agreeing here with the other guys, best to start with a fuel pressure check. I'm betting your problem is fuel related.