High idle-96' K1500 5.7 4L60e

[Jka251]

My 96' has a high idle issue that I can't figure out. So far I have replaced:
-MAF
-TPS
-IAC (x2)
-Vapor canister purge valve
-Cleaned throttle body
-Checked tension/sticking on throttle cable
-Checked for vacuum leaks

The truck acts like it runs in two different modes (for lack of better words);
First mode: Get in it and it has the high idle and shifts hard going from 1st to 2nd. No other hard shifting. Idle is usually 1000-1500rpm.
Second mode: Get in, has the high idle but shifts smooth as butter from 1st to 2nd but stalls and surges in the 40-60mph range; RPMs are up and down until I get over 60mph or mash on the accelerator and cause the trans to shift down. Idle goes up to 2000rpm (especially in Park).

Usually the truck "switches modes" each time I get in. i.e., runs 1st mode this time, next time runs in 2nd mode.

Had a reputable shop check for vacuum leaks and any other issue. Owner has a truck almost identical to mine. Did not find the cause. Could this be a computer issue? I have not messed with any of the fuel system (intake/intake gaskets, spider injectors, etc).

Any help is appreciated

 

[Sabinoerc]

My experience is with earlier years but first thing I’d check is fuel pressure and verify all the grounds are good/clean. From there, if still problems, fastest path to figuring it out something like this is to get datalog capability. Either a handheld scanner or diy TunerPro.