Having an issue at cruise in vacuum where the AFR's will oscilate between 10.0:1 to 14.8:1. Vehicle will misfire/stumble and go completely rich on the wideband. It only does this in vacuum and runs great as soon as it hits anything above 0PSI. Its really annoying driving long distances having to constantly play with the throttle to keep the vehicle from going full rich and misfiring.

I have tried leaning out the part throttle fuel tables and also the base fueling from 22" of vacuum all the way to about 2 PSI, but that didnt do much.

Vehicle is an 87GLHS, 87T2 electronics, running the 2.5L_MTX_Custom_v13 turbonator cal.

There are no codes being thrown.

Could this be a timing issue?

Any ideas on where to start looking?