Code P0507, 02 causing this?
 

So I had my MS (parallel unit) hooked up for over a year, and a few weeks ago the car started to idle high (1300-1500 RPM.) I got the code P0507 which is "Idle control system RPM high." So I was thinking this was my IAC. I removed my IAC and cleaned it, and I even put voltage to it, and watched it move.
Now my car is back to running on the stock ECU...ocassionally my idle goes high like it did, but otherwise it runs right around 1,000 RPM. I noticed that I never get a P0171 (System too lean) or P0172 (system too rich) code like I had been getting in the past. Could it be that my stock O2 sensor crapped out and perhaps the stock ECU is compensating by actuating the IAC?