Changed the throttle body and it's working the same as the original one. Initially, it was idling at 1500, and turning the speed adjusting screw all the way in (like the original) brought it to 1200.

I did find why I was having that surging issue. Apparently it was the new ISC; replacing the new ISC with the old one eliminated the surge. So now my idle starts at 1800 cold, and gradually goes down to 1200.

Summary of work:
1. Verified ISC is good.
2. Replaced engine coolant temp sensor
3. Adjusted and tested TPS/idle position switch according to FSM.
4. Verified ECU good.
5. Adjusted timing.
6. Verified throttle body good.

My last idea is to test all the MPI component harness connections to the ECU for shorts to ground or opens.

1990 Monty LS 3.0 A/T LWB. KYB front shocks, OME rears and coils. 33x10.5 BFG Muds. 5.29, 2.85, RD110, RD46. 2 bouncys. Aisins. Monte Disco swaybar disconnects. GenII t-arms, idler arm, UCA's, V45W front brakes. 2in BL.