Runs super rich under boost only

[The Monarch]

Hi, I am new to the forum and have a stock 89 Supra turbo.

My Supra started running badly, I can't tell if it was all at once or slowly over a few days because I have a slow commute in it normally.

The symptom is it stumbles and doesn't accelerate as soon as it is making boost.

We hooked up an LM2 wideband, and cruising and idle, the afr is almost perfect 14.7:1. Once it gets to boost, it goes to 9.4:1 and stumbles all over itself.

I disconnected the vacuum reference line to the regulator and that helped a little but it still goes to 10.2:1.

Where should I start now? Is the Supra closed loop under boost? Could this be the 02 sensor telling the computer it is leaner than it is and it keeps adding fuel?

Any other ideas are appreciated.

Thanks,
Hank

EDIT: I don't see any smoke while this is going on by the way. Could an ignition problem where the fuel isn't burnt in some cylinder act like this...show super rich? I think that normally shows super lean because of all the unburnt oxygen, but not 100% sure. Thanks