I need help from a pro!

[budstahoe]

Hi all!
I have a '99 tahoe LT.
About 8 mos. ago it started to run poorly. I was getting random misfire codes. then a few days later it wouldn't start at all.
I changed the fuel filter (cheepest way out first, right?) then i changed the pump relay. N.G. then the fuel pump......then the dist. cap, rotor, plugs, wires. I finally got the thing started but it would only run if I constantly pumped the gas! (misfiring really bad) after about 5-8 minutes of this it started to smooth out....But, still running poorly. At this point I gave up and brought it to a service station.
They checked it out and told me that the fuel regulator and the injectors (all 8) spider set-up needed to be replaced. I figured that I didn't have much of a choice and paid the $1800.00...Yipes!
At the time I left the shop the "Check Engine" light was on and they said that it should reset in a day or two. I checked it out and it had a code for the MAP sensor...But it was running well so I "let it be" ( I manually re-set it a few times but it always came back on the same day...

O.K. now to last week...Cold it runs fine but after aprox. 5 miles after it is warmed up It would start to hesitate in the middle of cruising steady? Next day, it became a slight bucking when I would accelerate a little quicker than usual like the on ramp of a hwy.. Then a day or so later it was bucking so bad I couldn't get it over 30-35 mph! seemed like it was chokink out?
I Did find that if I threw it in neutral and turned the ignition off then on it would some how "Reset" and run fine for a couple miles????? then start to "buck" again.
I figured I would check out the MAP sensor and found it was UNPLUGGED all along! so I plugged it in and reset the code.
As luck goes (mine) It didn't help. but I didn't get that code again.
I did get codes for it running too lean and the left CAT but I think that that was from when it would buck I would end up pi**ed off and floor it too much.
So, there I am back at the shop where they replaced the intake air temp sensor they said it was reading the air temp at -40f and they replaced the "y" pipes and the 2 CATs. I picked it up Thurs. Afternoon (They picked up $1200) and after about 7 miles I get that little hesitation again ARGH!

It is running better but I am still getting that hesitation, almost a buck after it is warmed up a few miles.
I am worried that the Bucking garbage is next again!

I need any/all ideas that could possibly make any sense out of this

Thanks in advance for any help that you can provide!
Bud