This may help someone in the future. My S1 VHPD developed pops and bangs on the overrun, felt slower than usual, felt like it had a misfire and sometimes had 5000ish rpm limit.
After some detective work it appears the problem is the vehicle speed sensor that feeds into the ecu, not the Stack dash. I guess it uses it to control stationary engine idle speed and gets confused with intermittent signals and probably messes up the IACV. I just cleaned the connector and all now appears to be well, running smoothly again.
My clue to the problem was I had always noticed the moving idle rpm is about 1600 and it drops to about 1200 when the car stops moving.