Another consideration
I had a very similar stumble in my 88k5. After getting some feedback on here I upgraded to the 3 wire o2 sensor. Not sure if this would solve your problem but it seemed that with the dual exhaust and headers that the stock o2 wouldn't send the proper signal if it wasn't hot enough.
So far I've had pretty good results. I had a very small stumble once after but no cel or issues for a little while now