Ok, I have a 98 GMC Z71 w/ a 350 vortec....it drives fine with no problems unless you "get on it"...then the rpms rev up but it won't hardly accelerate. However if I let off a little it will pick up speed. I noticed this on the interstate do around 65-70 but it also does it in lower gears too(but only when I "get on it". Any suggestions as to what this could be? My friend's dad had his dually put in the shop today for the same thing and he was told it's something to do with his fuel pressure, is this possible? Any help is appreciated.

You could have a partically plugged fuel filer.

GM had its share of problems with fuel delivery during the late '90's. The result in many cases will be that the trans downshifts, but the lean mixtures resulting from low pressures will cause the engine to lose power, and the trans will not upshift. Ray