02 Mv Question

I have a 2004 Mach 1 with a Kenne Bell 2.1. I have had the car for about 6 months and when I get out of the throttle it will stumble some times. I checked the 02 MV and it is all over the place. Cruising at 70 MPH it went from 90 to 700 MV. About every 5 seconds or so the MV drop to 90 and then jump back to 400 to 600.

It did the same thing at idle and the engine was smooth as silk. It does this on both banks. Any idea on where the problem is???

Thanks,

Barry