90 GMC running bad

I'm at a loss....I have a 90 GMC 1/2 ton 4 WD, 350, auto. Recently it started acting really strange. When I first start out it runs fine. After about 10 minutes it will hesitate,miss, and backfire through the intake when I give it a little gas. It is worse at lower speed and if going up any incline. At 55 - 70 mph it is OK, idle is usually smooth. Any ideas ? I checked the vaccum lines,plugs, wires, etc.
No check engine light either.
Help.....

First things that come to mind are:

1) Restricted exhaust (possibly clogged cat).

2) Too low fuel pressure.

If you drive it at night and park in a dark spot, does the cat glow red? If so, the cat is clogged. Typically, when the cat is clogged, it will get worse as the exhaust temp rises.

Unfortunately, testing the fuel presure is a pain in the butt since there is not a service port on the 90; however, it may help in diagnosing the problem.

//2000CAYukon

Replace Your ignition module.Garenteed to solve your problems!