I just had a similar issue tonight, but I have the L87. Driving southbound on I-15 from Vegas, headed to San Diego. While dropping down into Baker, CA, cruise control was set at 78 mph. Time was approximately 10:30pm. Outside temp was 95°. AC was off because sunroof and rear window were open. Engine temp was normal, oil pressure was normal, and trans temp was 147°. Just me in the truck, and no payload nor trailer. Southbound I-15 dropping into Baker, CA is a steep decline. Iirc, 6% for several miles. The mountain pass parks at over 4,000 feet, and Baker is below 1,000 feet. Due to no reportable issues with the truck I continued to drive, but immediately closed the windows, turned on the AC, and pressed the OnStar button. OnStar ran a check for codes and didn't find a single one.
One thing to note: this is the 2nd time I've had an issue with the truck on this exact same stretch of highway, and both times it was 90°+ outside. Cruise is always set at 8 mph over the posted limit, and it only happens while going downhill. I've made this same trip several times over the last year, but outside temperatures were much milder, and nothing happened during those trips.
Imo, no point in taking the truck to the dealer since they won't find any codes to help them. So I guess that just leaves me to being curious what everybody thinks. If it's happening to both the L84 and L87, then it's probably associated to something common for both engines.