350 tbi idle surge then dies off. help!!
 

90 chevy 2500 w/350 tbi rebuilt engine. when cold it starts but wants to die if you dont keep tapping gas pedal after a minute or 2 it will stay running but revs high then low almost to stall then back up to higher than normal rpms . have new map sensor, 02 sensor, engine code 33 map sensor too high. test drive it would spit and sputter until it got up to about 20 or 30 mph then it would run much better. trying to go up a small hill it has no power, but on level pavement at about 30 theres no hesitation at all almost runs perfect. timing is right on, no vacuum leaks,. did ohm/volt check on TPS seemed ok. I know it's not an internal problem . is there something else I should look into or test? Ive never taken a vehicle to a shop to diagnose. would rather find it another way. It seems it should be something simple to find. it is starting to annoy me and I need it running good before summer starts which is only a few weeks away here in arizona. any ideas would be appreciated .

i had a problem like this, on an old dodge aspen, my problem was a clogged muffler. if you stepped to hard on it, it would have a raspy sound and no power. if you let it get up to about 30 on flat surface it would be just fine after that. try smacking the muffler with a rubber mallet, not to hard but a thunk. this might free up a clog. take a quick drive and see if it helps. if you hit too hard you will mess up a good muffler so again not to hard.