HELP - Loss of Power and Shifting Problems (No Engine Light)

Hi all, I'm new to the forum and hoping someone can help me... PLEASE!!.

I have a 2003 Envoy with the l6 and we are having major issues. It has a major loss of power at full throttle or under heavy acceleration. Normal driving is fine but when I mash the pedal to WOT it may or may not go into passing gear. When it does go into passing gear the RPMs jump to 5K but it won't up shift until I release the accelerator a bit. When it doesn't downshift into passing gear it makes an odd sound like it's starving for air and slowly builds RPM's (almost seems like it builds power in small stages at a time). My wife stated that when she has driven it there have been times that the it will feel like the accelerator is pressed down when her foot is off of it and it want's to go (as if the throttle is stuck) I have not been able to duplicate this.

Things that I have done thus far..

Fuel Filter, Air Filter, Fan Clutch (major pain in the ass), checked the exhaust for restrictions (doesn't seem to have any), cleaned the throttle body, and a full tune up. NONE OF THESE EFFECTED THE PERFORMANCE!

I read somewhere that there may be a PCM flash update I'm not sure if that will help or not but I'm running out of ideas and I have a hunting trip planned in a couple weeks.

Any suggestions would be greatly appreciated. As I said before, there are no trouble lights lit, I did have one for the cooling fan a few months ago but I just replaced the fan clutch this morning.

Thanks,
Joe