Intermittent power loss in an 04 CTS?
 

My father has a preowned CTS, still under warranty. He's been experiencing an 80% power loss on infrequent occasions while driving. He'd stop, turn off the engine, turn it on again, and the problem would go away.

He's taken it in to two separate dealers. One said they couldn't figure out what was going on, since the problem left no diagnostic code. The other suggested that it might be a bad harness of some sort, which, a couple years ago, was cut and spliced for some unrelated warranty work. The splice might cause it to periodically send current at the wrong voltage (or something) to a computer somewhere, which would trigger the power drop. That dealer, however, couldn't get authorization to replace the harness under the warranty, since they couldn't recreate the issue.

Do any of you guys recognize this? Is there a way to force the recreation of the issue somehow, or maybe an easier fix than replacing the harness?

(This is in an 04 CTS with the automatic transmission and the 3.6 L V6)

Thanks in advance.