Driving my 850 the past few days and since everything it has been running great on the new ARD tune. All of a sudden, driving today the car will barely accelerate. Basically hits a wall. I checked the plugs, the gap is still fine and they only have about 100 miles on them. Relatively new cap and rotor. No boost leaks, no vac leaks, everything looked good.
Went back out and watched the AFRs while this happened. I was essentially getting reversed fueling. When I accelerate, the car goes stupid lean and won't move. I let off the gas and it goes super rich for a few seconds before going lean like it should between shifts. I haven also been getting horrible gas mileage.
Before the recent additions (greens, meth, turbo rebuild, and ard blue tune) the car ran great, no issues. And it has been running great for the past 5 days as well, this is literally mid-way home form work it started doing this. 
Any thoughts? I'm leaning towards an ECU related issue at the moment.