Car stumbles or dies under load
I noticed this problem the other day, I turned the radio, defrost, headlights, and wipers on and noticed my dash lights started to dim. Then going down the highway it acted like it was gonna die. So I press on the accelerator and it ran fine the rest of the way home. Well I took it to autozone and they said the battery and alternator were both good. So now I'm back to square 1. And I'm not sure what else to look at.
Check the belt on the alternator. It might be slipping and causing a low voltage condition with the accessories on. Next time it happens, try turning some things off to see if the condition improves.