check the 02 sensor was not damaged during installation of cat bypass
I have the same MY and have a fluctuating idle as well. Also have a cat bypass w/stock muffler.
My chip is not SW, but the Russel Berry chip by MAXhpkit comes in 3 standard forms depending on exhaust type, fuel type, and air meter.
I'd look at your throttle assembly in the engine compartment to see if you are suffering from this problem
http://forums.pelicanparts.com/porsche-911-technical-forum/541862-big-scare-today.html