2002 GMC Savana 2002 vortec 5700 V8 sfi engine problems

I bought this truck new and started having issues in the first year. The dealer claimed they could not replicate the problem. The service engine light would come on when I was speeding up to get on a highway or going up a steep hill. The light would come on as it slipped into the last gear and the truck would shutter and loose power and act like it was going to stall. No back firing just loss of power. I brought it to a non dealership mechanic after it was out of warranty and he pulled multiple trouble codes for mis fires on random cylinders. These were the only codes recorded so I guess the dealership forgot to tell me this. My Mechanic did a complete tune up and then replaced the fuel injection system(spider) with a new one(new style). It seamed to solve the problem for a while but now it is even worse than before. What could the problem be?