1992 isuzu trooper 3.2 DOHC 4X4 LOW IDLE GOES UP AND DOWN 750 RPM TO 1500 RPM

4psi idle (bad) and revving to 3k does what, you didnt say.
what was idle RPM 800 or 400>?
the answer is different by hot and cold oil
and oil type, no type stated (do not say brands) 10w30?
running 0w are you>?
and is as low as 8psi hot. at idle ,that is if idle speed is at spec.
is it?
and 3000 rpm would be like 25psi hot, and more cold.
the max pressure will be the pump regulator value.

here is the sensor spec.

The sensor closes when the oil pressure drops to between 4-7 psi (28-48 kPa).

the oil type and bearings clearances (in total) dictate pressure.

3,1 CPC engine spec is.

Temperature sensor low antifreeze may need bleeding ?

I doubt VERY much just replacing the converter is going to solve that problem. Get a second PROFESSIONAL OPINION.

Sounds like an Idle Air Control (IAC) motor to me.

check for a vaccuum leak this is where i would start if it rolled into my shop. possibly a PCV valve sticking or a hole in the hose, this hose should run from the valve cover, or crank case and then to the intake, or carb.

Do you have a check engine light? Which engine? My 1996 Trooper with a 3.2 D.O.H.C. is awful for plugging up the E.G.R. I have cleaned it several times! Good Luck!

Normal...

The RPM is token at the alternator.
When the belt slips, the alternator turns at lower speeds too, and thus the RPM shows low.

Change those belts, quickly!

Sounds like the tps (throttle position sensor) is either defective, or out of adjustment. Need an oscilloscope to check, so see a mechanic. Could also be vacuum leak. check lines for cracks and leaks

Isuzu changed trooper models in 1998-99 if yours is the older type then 1992-98 if the newer type then 99-05

Sounds like the ISC (Idle Speed Control) which is adjustable.  The secondary idle needs adjustment to prevent the car from wanting to stall at low speeds.
Greg