What will cause my car to run rough with little power while in reverse or drive when in park it runs and idles good but it takes a long time to get up speed.

You personally can not test an ecm

Very few professionals attempt that,
as it doesn't fail often & it takes very little
to smoke one, trying to test something you
know little about even with wiring diagrams

Do you not take the car in a couple times
a year to a shop you have used for years
with good luck?

Is the check engine light on ?
Have you checked the computer for trouble codes ?
You could have one bad spark plug even with only 30k on it.
Problem could also be a dirty injector.

Clean ur throttle body takes 30min cleaners only 8$ at aurozone and there good how to videos on you tube my truck always had rough idle bur got to wear if I stopped withoit my foot a little on the gas it woild die it had me worried but cleaned thrortle body it runs perfect

o-2 sensor

Sounds like the Torque Converter Lock-up Solenoid is failing. It will cause the car to run rough at slow speeds, and sometimes stall while idling in gear (at a stop sign for example)
To test this, locate the 2 or 3 wire connector on the left side of the transmission (looking at it from the left side of the car, it can be hard to find some times) Disconnect it. If the car runs and drives ok with it unplugged, the Lock Up Solenoid has gone bad. The solenoid itself is inside the valve body, inside the transmission (above the transmission pan)

Your ignition coil might be bad. If it is happening after it rains, check the distributor for moisture. My ignition coil went bad on my 98 jetta.

If you replace the ignition coil, it runs rough for a little bit and it needs to sync. Just drive it around a little bit and it should steady out. If it doesn't take it to a mechanic.

Good luck.

COULD BE GROUND PROBLEMS OR THE TACH GOING BAD.EVEN THOUGH CAR SEEMS TO BE RUNNING GOOD IDLING GOOD.SOME TIMES A SPARK PLUG MISFIRE.CAUSING A LITTLE ROUGH IDLING CAUSING TACH NEEDLE TO GO CRAZY.I SEEN ALTERNATOR WITH VOLTAGE REGULATOR GOING BAD CAUSE A POWER SURGE CAUSING HEAD LIGHTS TO DIM A LITTLE THEN BRIGHT WHILE ENGINE IDLING.THIS WILL CAUSE TACH METER NEEDLE BOUNCE AROUND ALSO.

has it had a tune up?

I had the same problem with '99 GA I changed the idle control valve , solve the problem

low idle speed...or the need of a tune-up