Slight surging while decelerating / coasting

[stuntmanmike]

Hey guys, noticed that my 90 tends to "surge" slightly when coasting and decelerating in gear, happens in any gear around 2400 rpm and again around 2200 rpm. It's usually accompanied by some light popping of unburnt fuel in the exhaust. (I have a borla cat back and radio doesn't work so I'm listening closely!) Above 3000 rpm car has a smooth, linear decrease in engine speed and seems that the computer or some sensor may be ever so slightly increasing RPM or changing timing or something below 3000 rpm?

Runs very smoothly at idle and no issues at all when on the gas. The car has replaced injectors and I put a Haibeck chip in last weekend...but pretty positive the car was doing this before chip went in. (it is nice to not have the CAGS and notice a quicker return to a lower idle!)

I'm new to ZR1s, is this normal or do you think something is up?