I recently installed a vf39, 440's and front mount onto my 93 legacy turbo wagon. I'm adjusting fuel with a HKS super afr.

The hiccup/fuel cut (I think this because my wideband goes lean for a split second afterwards) happens every time I go past 0bar (even just to .01bar on my apexi avcr) and come back into vacuum. its very noticable, passengers think my car is breaking. :(

Has anybody ever run into this?

It sounds like your in safe mode.
Have you checked for a code?
Atmospheric pressure sensor (Map sensor) code?

I thing I misunderstood. So you can run boost it's just bucking when coming back to vacume? If that's
the case it sounds like a blowoff /recirculation valve issue.

The stock ECU does a amazing job adjusting for the 440cc and vf39.
Only problem I had was at WOT 14-15psi I noticed rich - AFM over 4.8V? = 100% duty cycle.
I'd suggest that you let your ECU adjust to the injectors and turbo without much tuning for about 10 days without flogging it too hard and watch your AFR.
Good luck :)
Eric

I don't think that I am in safe mode since I can still boost to .9 bar.
I know what the car feels like when the MAP is dead, that happen a few months ago.
Could this be compressor surge? I think the hole I drilled for the BPV is too small and not venting very well.