2000 GMC Jimmy Check Engine light

For the past year or so I've had the following problem. Often, when accelerating uphill such as when passing, my Check Engline light will come on, I will lose power and RPMs will drop. The engine seems to die but will "catch-up" with itself after about 5 seconds and run fine again. The OBD codes indicate that an engine misfire was detected. The fuel pump has been replaced, they have run fuel pressure tests, given it a complete tuneup (plugs, wires, etc.), checked all sensors, etc., but the problem continues. Someone suggested a clogged return fuel line so I've began using fuel additives, which may be helping. Any other ideas? Many thanks.