You may be fuel starved by a clogged fuel filter, but more likely, its a clogged exhaust system. The likely culprit is the catalytic converter.

Run a vacuum test. If the exhaust is severly restricted, the vacuum at idle will be low and may wander in its value. If the exhaust is not clogged enough to affect idle, other checks will be needed to confirm a restricted exhaust.

A resticted exhaust fools the engine controller into thinking the engine is under heavy load because the vacuum is down and the throttle is open more than normal. An over-rich mixture may result with retarded spark timing, all nasty on performance.

Reduced vacuum will reduce the amount of PCV (positive crankcase ventilation) to the extent that crankcase vapors may backup through the air tube into the airbox and air filter. Look at the air box and filter. Is there oil in there?

If your loss of power comes on gradually, or manifests itself when pullng a long grade, check the fuel filler cap. On modern, sealed fuel tanks, the gas cap has a one-way valve in it to let air into the tank, but prevent fumes from escaping. If this valve gets plugged, or the cap has been replaced with the wrong cap, a vacuum may be forming in the fuel tank. [One time, while driving a borrowed pickup truck, I ran into this mystery problem in the Smoky Mountians. A close exam of the gas cap showed that it had been dropped in the mud and stepped on, then nicely cleaned, except for unnoticed vent which was plugged. The problem did not show until 600 miles from home. The temperature dropped while climbing the mountain and caused the problem to show.]

One easy test is to remove the cap when the problem appears. If considerable air is drawn into the tank, it had a vacuum; If nothing happened, the tank was a atmospheric pressure; and if air/fumes hissed out, the tank was pressurized. If it was a vacuum problem, loosening the cap and retightenning it will elimate the problem until vacuum builds again.

In severe cases of vacuum, the tank may collaspe.

I did find a small leak from the return line from the throttle body to the tank,,repaired it and didn't help..I checked the gas cap and heard no hiss noise..I did advance the distributor some and it seamed to help the problem..It still idles smooth even though I advanced it about 8 degrees you can't tell the diffrence..It starts fine and all seams good...What I dont understand is there should be some increase in the rpms from my advancement,,It still idles at 800 rpm in park and about 600 in gear..Unless the computer took over to control that..Any ideas about this??Like I said it idles fine without a miss... I also took off my exhaust at the cat..The pipe that ran into my muffler had part of my cat pieces in it..Large chunks..They look like little behives and was big enough not to fit into my muffler and I suspect it did cause flow problem..Pieces like 2x4 inches long..But that didn't help the problem either, when I put it back together..I did make sure the cat wasn't clogged... The timing seamed to of fixed the problem..

Let's see, you have a '91 350 V8 with throttle body, Suburban. I'm can't be sure about the setup on these, but I think it still had a distributor, and the timing is taken from a sensor in the distributor. There may be a cam position sensor as well. If it as I remember, there is a timing mark on the vibration damper and a degree plate bolted to the front of the engine or cam chain cover.

You mentioned that advancing the timing helped - a clue!
Are you sure that the timing mark is in the right place? it's on the vibration damper and these have been known to debond and slip! To test, remove no. 1 sparkplug and position (by hand) no. 1 piston at top dead center, then look at the timing mark. Is it at zero degrees within a couple of degrees, or off the mark. If its off the mark, there's a problem. I have seen these as much as 15 deg off, and still running at idle. For a temporary fix, put a new TDC mark on the damper with white paint, white-out, or other convenient marker and re-time. After this, don't trust the timing mark until you have effected a repair. Once the damper slips, it will continue to slip.

After you get it timed properly, do your own tune-ups until a repair has been done. Each time a stranger does a tune-up, the timing is in danger of getting reset to a bogus value. Once set, it has been my experience that the timing does not change until the sensor is changed, and then the change is minor.

Check the paint color on the vibration damper and the valve covers of the engine. The colors should match. These engines went into vehicles that had the timing degree plate on either the left or right side. One was painted red and the other blue. I had a friend who intalled replcaement damper but with color mismatch. He had awful timing problems until the problem was discovered.