Could it be regular oil not synthetic mixture?
I bought used 2011Chevy HHR from dealership with 80,000 miles. It started stalling at lights. Something seemed to drag on the engine; especially if the ac was on. No engine repair engine light on. Engine would die at light when I accelerated to go at green light. Then I went to get oil change and the guy told me Chevy HHR used synthetic mixture specifically. I got the oil changed and now the engine runs great. Could it have been wrong oil causing this?