Air Fuel Ratio problem

Hello All,

My first post on this forum.
I have a 2004 GTO with a LS2 forged 402 CI with twin turbo's on it.
Borla exhaust. Just a quick street car. Yet to race it.

The problem I am having I need help with is....

I have a wide band as part of my gauge package.
At cruising speeds the engine suddenly goes lean 17-18:1 for just a second.
Enough to throw a P101 code, every single time I take the car out.

Accelerating, no problem.....12 to 11:1
Idle 14:1
Cruise 13.5 to 14.5...then the lean spike.

GT35R turbo's, twin walbro fuel pump. Fuel pressure stays contant.
I am running a MAF setup. SLP MAF...might be the problem.
Cannot be tuned out so far. PE tables etc. Nothing works to get rid of the problem.
I am not a tuning expert but know a little. Pro tuner I paid gave up.
Also my  O2's are aft of the turbo's in turbulant air. 6" downstream of the turbos.
No room to place the O2's before. Manifolds dump right into the tubo between 3 and 5 and 4 and 6. Its a tight fit. Manifolds are cast iron.

Any thoughts on the cause of the lean spike would be greatly appreciated.
Thanks
Mike