Well, I'm making progress. GM Dealer gave me bulletin PIC6110B: Timeshift Audio Playback Choppy Distorted. Says "Customers may comment that intermittently Timeshift audio is choppy, distorted, skipping, or garbled.
"RECOMMENDATIONS: Please do not replace any parts or make any repairs."
"Please communicate to the customer that this condition is a normal operating characteristic of their vehicle."
What a bunch of bunk! GM can't won't fix it!
Effects: 2015 Buick Lacross, Regal, 2015 Cadillac ATC, CTS, EL, EScalade models, SRX XTS, 2014 Silverado, 2015 Silverado, Colorado, Corvette, Impala , Suburban, Tahoe, 2014 GMC Sierra 1500, 2015 GMC Sierra, and 2015, GMC Canyon, Yukon Models.