but you also mention engine's been modded some, and the symptoms you mention can also happen due to a vacuum leak, such as large vacuum hose to a brake power booster, or a manifold leak from around mounting gaskets. If .latter, be sure to use composite gaskets instead of solid metal type.
I have a 2009 chevy cobalt lt 2.2 litre engine. It has many mods that were not done by me, I'm the second owner. It has 105000km. It has a supercharger, coolant for the supercharger, some gauges that aren't hooked up, fully custom exhaust. It was essentially modded to be a Cobalt SS. The problem that I'm having has been going on for some time now. Whenever I'm driving and depress the clutch fully to make it go into an idle state, at first the RPM will bounce down to around 500-600 rpm and come back up to its normal idle(around 750-900). Sometimes itll bounce around in the 500-800 range and come back to normal. When this happens i can audibly hear the change in sound from the exhaust, sounds like its close to stalling. I've tried cleaning the throttle body, even completely replaced it, changed plugs and just some general cleaning around the engine bay. I don't think there's any vacuum leaks. Nothing noticeable anyway. When just idling the car runs completely fine, it's only when i depress the clutch completely when coming to a stop sign or something that it bounces around. It appears that it's getting worse, only thing i can think of is that it might have a bad o2 sensor. both the o2 sensors are pretty jumpy as far as readings go, im not sure what they're exactly supposed to read anyway. Could also possibly be MAF sensor related. Thanks