Lean AFR Spikes when cruising

[whiplash willy]

I have a stock SR setup except for exhaust, walbro fuel pump and nismo AFR (32psi at idle) I cruise at about 13-13.5 AFR. Every 30 to 60sec my AFR will spike to about 17 AFR and hold for a couple of seconds, then go back down. This is while I am holding a steady throttle. Even if I step on the gas during this spike it will stay lean for that couple of seconds before it goes back to normal AFR. What could cause this?