Check the color and level of the ATF. You might want to dump the old ATF in the sump and refill with the same amount. If you drain the old stuff, I suggest doing it with a cold tranny that has sat for a few hours. That way, any heavy sludge will pour out along with the real dirty stuff. You will need a funnel with a small opening that fits on the ATF dipstick, and be careful as adding new ATF is a slow process.

Because ATF expands when hot, you might just be a little low and the internal pump is cavitating and lowering the pressure for a few seconds.

As to your fluctuating idle, you might need to replace the PVC valve.
--
My name is Klaus and I am a V♂lv♂holic

I think this car has the CIS system. My first thought is the air flap sensor below the throttle is sticking during the low volume of air flow.
You may need to check for that which is under that big rubber bellows.

Also check for any access air that may be passing around that flow plate like the idle air bypass screw or the auxiliary air valve used during warm up. These two things could out of adjustment.

If after warmup the engine runs good then forget the air flap. Most of the time they do work fine.

Like I said, the flap was my first thought.

Phil