2008 Chevy impala, speedometer fluctuating and then car stalls

I have a 2008 Chevy impala, I have taken it to multiple shops and they just tell me they have no idea what’s wrong, im to my breaking point because it’s so stressful to drive it, the speedometer fluctuates and then the car stalls at stop lights and stop signs, but here’s the thing it only does this when it’s hot outside, if anyone could please try and give me some insight it would be greatly appreciated