computer or sensor or....?

[scratch22]

OK, I've got a stock '99 GMC Jimmy. Everything was great until about two weeks ago, I thought my tranny was going bad, because I turned the truck on,went to drive it and the RPMs wouldn't stay steady and the truck kept on trying to shift even when it didn't need to. Anyway, I drop it off at a tranny shop,they have it for a day before they can look at it and when they do, they say that it is just sluggish because the cat.converter needs to be changed (which I was getting ready to have done). He says that he put it on the computer and it was showing no codes and that my tranny was fine. I pick it up and low and behold, it is driving fine. I get the cc changed and about 500 miles later, I drive into work, turn the truck off, then turn it back on so I can go get some breakfast and it's doing the same damned thing. This is several weeks later. I send it to a different tranny shop, it sits there overnight, he looks at it the next afternoon, and it's running perfect! No codes! I think back and it's almost like the truck was out of gas, RPMs drop, then had gas, RPMs go back up, but before you reach cruising speed, the same thing. Up and down and you can feel it going into gear. I'm nervous now that at any time, I could turn it off to run into the store and come back out to a truck I can't drive...Are there any sensors that could be failing and cause this? Maybe I'm not going to get the code until I get it read while it's doing it? I am female, but I do know how a car works and I am tired of people getting dollar signs in their eyes when they see me...Want to have a general idea of what it could be before I take it anywhere else. I know the basics, but this electronic stuff is very frustrating. Any help would be greatly appreciated!

[Fireplug]

Fuel pump or clogged fuel filter

[MDT1955]

I've got a '98 Jimmy SLT 4WD, here's been a few of my problems

#1.) Gauges jumping off and on then going completely dead...except
for Oil pressure and Ampmeter gauges.
#2.) When putting transmission into drive, transmission slips on takeoff
in 3rd gear. In other words NO LOW (1st), 2nd, or overdrive. All
you have is 3rd (drive) !! Will manually downshift to 2nd, that's all.
Check engine light is constantly on....the code # meaning?
"Transmission is in Safe Mode". After 1 year, it has cleared up on
it's own. Did I drive it like this for 1 year? YES I DID.
#3.) Windshield wipers refusing to work and when they do, do not stop
at the base of the windshield.
#4.) Headlights refusing to work with or without Daylight switch.
#5.) Interior dome lights sometimes refuse to go off with timer or
manually.
#6.) Interior lights sometimes refuse to come on.
#7.) Heater and AC alternate hot and cold air every 15 seconds no
matter where the controller is set. (computer climate control).

A friend finally traded his after dealer replaced ignition switch 3 times.
Twice AFTER warranty was out at 36,000 miles. I believe ALL my problems are with the computer....G.M. moved a plant for the wiring harness to Mexico in late '97 I believe. I AM a G.M. employee in Indy.

Any clues or simular problems?

[smonaldo]

Mine is in the shop now....had the same problem which I posted here. RPM surges, etc. Just got the call that they believe it is in the 4wd low unit, computer is being told that it is in 4wd low when it is not, and thus it tries to limit rpms, shifting, etc. Mkaes perfect sense to me. They are diagnosing further to determine if it is in electronic unit or wiring short.