Most UPS's and inverters are simulated square wave, most devices are fine but things such as PCs can/will overheat or fail prematurely when run off a square wave. If you are powering off AC to a power adapter (back to DC), I'm not sure if this would be an issue but almost everything runs hotter off a square wave. If I were to guess, it was probably cutting out last time because your overheating... all the extra heat generated in the cabin on the track+extended period of use = more heat. Even my GoPro has had this issue... seems fine in the paddock but will sometimes have issues on track, I've noticed it'll be hot to the touch whenever it stops working.
Last time you tried a USB adapter, I think it may just be an issue with the cigarette connection... they aren't the most reliable way to transfer sustained current in a high vibration environment. Have you tried something like this that hardwires in? https://www.amazon.com/Boostable-Charge … &psc=1
If you want to go the AC-powered route... we use this true sine wave inverter in our suburban (https://www.amazon.com/gp/product/B07Q6 … &psc=1) and it works great. It looks out of stock now but they're all the same. We use it in the truck when we have a long tow (like when we towed half-way across the US for the HPR 24hr) and powered a laptop, blu ray player, and TV for entertainment on the back 3 rows and it didn't skip a beat.
Full Ass Racing
#455 Piñata Miata - 1990 Miata
#735 BMDollhÜr 7Turdy5i - 1990 735i