I have a 96 c1500 with the 5.7 vortec, the truck has trouble shifting and cruising at high speeds. it accelerates fine to a certian point then starts to sputter and backfire and has trouble shifting.

check the rpm and road speed when this happens . The cruise control will increase to keep RPM constant but as the road speed drops the auto may be shifting down and as the rpm increase it shifts back before the rpm adjusts. You can override a cruise control in this situation by simply pressing down on the accelerator to maintain the power setting and road speed and when the situation is passed and you take your foot of the accelerator the cruise control will return the system to the set position without you needing to do anything

If you have trouble in fixing your vehicles, try this site. Very helpful
reliable-store com

i think your clutch may be failing and on its last leg.

That sounds like a bad tourqe converter.

sounds like you are suffering from low fuel pressure, with a proper gauge you can check to be certian that you have addaquit pressure. at idle you should have 14-18 psi.

could be a lot of things. I personally would start with your vacuum lines... check for a leak somewhere. You can do that by spraying carb cleaner (if motor tries to die, you found your leak), or it could be a sensor, likely idle air control, mas air, etc.)

With normal acceleration the 1st to second shift point should be around 2500-3000 rpm. Most of your shifts under moderate acceleration will take place at this same point or when there a light load on the transmission. Under hard acceleration the RPMs will climp higher due to increased power needs and the computer will shift when less power is needed or the RPMs climb to an unsafe speed say5000-6000rpms (depending on where yours redlines). Also going up a steep hill will raise the RPMs higher and goind down a hill will cause a faster shift at lower RPMs. These can always be altered by an individual owner by adjusting shift points, using performance parts and if you have problems (low tranny fluid/trans issues). Your trans RPMS are fine and will vary depening on how your foot accelerates. The shift pattern should be the same every time you drive slow/medium/fast/, look for changes in those patterns you will also catch a problem early.

This could be a kick-down servo adjustment.

Refer to the service manual.

Frank
Frank's Friendly Cars Maui Car Rental
www.MauiCarRental.biz

have you had the transmission flushed? it seems to be caught in a limbo. where the calibration of the speedo and the transmission are not compatable. Now as wierd as that seems it happens usally like when going up a hill if you have a manual trans. and cruise locked in. it cant hold that speed and moves to a different speed if you dont down shift. Or perhaps your cruise control cable is slipping. is it vacume over mechanical? pop the hood and make sure all your hoses to cruise control are tight and no leaks.

I had that problem with my 96 blazer , than i smelled burning for a while ! It was my transmission!