Ktmscott here in San Antonio texas
My 99 xk8 4.0 started surging (bouncing) at idle when cold. Once warm Car runs great but concerned I may be on the verge of something bad. Any ideas out there
thanks in advance......

 

If you’ve disconnected the battery, Surging at start up does happen when you re-connect the battery. The car is relearning the idle speed. This can be prevented by turning key to on, not start, wait a few seconds, turn off, wait a few seconds then start.

Otherwise you may have a battery and/or cable problem as the ECM is losing memory.

 

A wild guess, it could also be a vacuum leak. Mine did that and I found a vacuum hose split. Good luck hunting it down.