90 5.7L Stalling in reverse

I have a 90 5.7 TBI that is stalling in reverse only. Everything else works great. It stalls only in reverse and only after it has reached operating temp. The RPM fluctuates and then dies. I'm stumped.

Solved it. Once I had the filter and the spacer off I could hear the IAC valve stuck open while in park. I put into reverse and it was much worse. I replaced it and reset the ECM and shazam; it was fixed. I still don't know why it only stalled in reverse and not any forward gears.