Idle bounce like you've described is almost always certainly a vaccuum leak - caused by a hose with a hairline fracture in it.Originally Posted by pong
It's bouncing because as the icv is opening, more air is being drawn through the crack in a hose (between the MAF and ICV, due to more vaccuum)- causing the icv to close again as there's now too much air. Unplug your MAF with the engine running and see if the idle stabilises - if it does it's a sure sign one of your hoses is cracked.
If so, take all of your air hoses off the vehicle and flex/bend them around a bit - any cracks will immediately show up.