Tbi 350

I was given a 1990 chevrolet cheyenne. 2500 and
finally got it home. It had sat for about 6or 8 years.
I put a battery in it, new fuel pump and filter and
primed the throttle body and it started right up, it
idled well considering the circumstances. I was
able to drive it across the subdivision and back,
about 4 miles with no problem. And again the next
day just a block and back. On the second, short trip
it began to be difficult to keep running when in
gear. (it's A/T btw)  gradually but quickly it got
worse. Now it would not run unless I was
constantly and gently pumping the throttle. I have
cleaned the throttle body, tested the iac and tps
with a multimeter, adjusted ignition timing, and
looked for vacuum leaks with starting fluid. After
the timing adjustment I was able to get it to idle
but it was mildly to moderately rough. I thought I
was on my way but when I put it in gear it begins to
sputter and backfire. I can't even get it 30 feet
before it dies. It does not jump forward forcefully
or jerk it just idles down and then spits and
shutters till it dies. It will start right back up but
usually afterwards it USUALLY will not idle. It
seems there is little difference in the engine temp.
I'd like to get some some ideas for troubleshooting
but I have very basic tools and no access to most
tools needed for testing (i.e. Vacuum pump,
pressure guages, etc.) any and all useful
information will be greatly appreciated.  

1990 cheyenne 2500 5.7l tbi 2wd service bed