Did you adjust minimum idle air and TPS voltage yet? These may have a big effect on the stalling issue.

You also might try installing a heated O2 sensor, there are discussions on the S-Series.org 60 Deg V6 board regarding this. The heated sensor keeps the engine in closed loop longer and the hot sensor provides a more accurate air/fuel input to the ECM.

If your speedo needle is wavering at slow speeds, you might try pulling the drive cable out and lubing it up. A wavering speedo sends a goofy signal to the VSS buffer which causes some Very Strange Stuff. My old '90 2.8 was running funny and throwing Check Engine lites until I lubed the speedo cable.

What kind of spark plugs are you running, if Bosch throw them away! These motors like the AC-Delco Rapidfire plugs, the standard (non-platinum) ones are cheap enough and work very well.

Check your EGR for deposits/sticking as a partially-open EGR at just-off-idle could cause stalling.

HTH and let us know what you find...........ed