2003 CTS cadillac, 135k miles. The idle jumps from 1000 to 1500 continuosly then it climbs up the the rev limiter for 10 seconds and repeats over and over. The trans now shifts hard, the dash says "service idle control" and the fault code says "*ECM" Is this a bad ECM? It's happened before for a couple days and then went away, but this time its not going away.. Any help is greatlt appreciated

I have the same problem. The dealer has installed a new throtttle body, then a new ECM. I STILL have the same problem. The dealer has no ideas and now wants to charge me to see if my valve seals are leaking. Sure, after $2,400 I'm going to let you pull the top of the engine off to check my valve seals.
One mechanic (non-Cadillac) feels the battery ground wire is corroded and should be replaced so that's my next step.

The service idle control and ECM problem should be addressed. I would try a used ECM from the salvage yard.