no get-up and go

Over the past few months I've have this problem of no power. I have a 1995 pontiac transport,3.8 litre,273,000klms. Starting with today the fuel pump and screen and fuel line filter been replaced, new plugs and wires replaced and new exhaust system. I've also had 2 out of the 3 coil pack replaced and new air filter. The van idles smoothly until you try to pass another car that when you notice no power, it takes alitte time for it to pick-up speed. My mechanic can't figure this problem out, so far its cost me a bit of money replacing these parts, the only item I haven't done is fuel injector cleaning. I'd really appreciate any info regarding this problem and how to fix it.