RPM raises/ MPH Doesnt. Tranny? Tranny converter? Fuel Line? Gear shift?

I have a 2003 Saturn Vue. Ive taken it to 2 different Transmission specialists as the rumor with so many tranny failures for saturn vue 2002-2005 owners. All said no problem with Tranny.
1. Doesnt do every time but sometimes if I go from idle and build speed as I begin to go it feels as if I dont get the speed I should be getting.

2. RPM gauge is climbing as im pressing the gas, but mph doesnt rise either.
( My meager knowledge on Engines would lead me to think its not an Engine problem, if the car is delivering the RPM's, the next thing is the Tranny.)
a. I.E. If I am in a parking lot starting from Idle to accelerating to leave the parking lot. ( Mostly it happens before even leaving the parking lot) So maybe be jumping from 1st to 2nd, HIGHLY doubtful it happens on the way to 3rd gear) Now Im speaking on behalf of someone else now, and I encouraged her to try to " feel out and listen to " the car shifting gears, and watching the RPM gauge to look for the Spike and see if the car problem happens while the gear is shifting, ( ill get back with ya on that)
3. Only happens from acceleration from low speeds ( when it does happen)
4. Ive heard possibly transmission fluid leaking, to maybe the gear shifts are sloppy or delayed.
1. But if its an automatic I read they dont have gear shifts per se, but the trannys computer kinda does that function with a converter so maybe a converter problem?

5. Regular driving doesnt have any problems.
6. Even more bizarre it tends to happens mostly in certain spots.
I.E. The same places she visits on a regular basis
One in particular has a inclined slope when you want to leave, hence she has to hit the gas a little harder ( im sure as you know the Saturn Vue doesnt exactly have much Giddy-up)

6. Its so intermittent that only ONE of the mechanics have been able to successfully RECREATE the problem. Weve let different mechanics have it for several days up to a week and no problem.
Finally when one mechanic got it to happen, he said the machine told him it was the O2 sensor, he switched it out, ran great for a week. Did it again.

7. Many times when she goes to leave her work ( not really on an inclined slope but does need to go from 1st to 2nd.) The same thing will happen, and shell just kinda take the foot off the gas, or kill the car, then ride around in first gear and then its fine.

8. The car has never completely died out during this stall, just a momentary lapse of no-go.

9. ( Not sure if it ever happens, while gear is shifting, she didnt know to look for it, but she knows to look for it now.)

any Advice?