After running in park with a/c on,car would die when put into drive. Why?

What is your question?

This is now the second time in 2 months similar issue. Was at stop light, went to accelerate on green and car died. Was able to get it started but felt like it was going to die again. Mechanics found nothing wrong. Yesterday, I had car sitting in parking spot with a/c running for about 1/2. Put it in reverse, it died. Started back up, put it in drive it died again. Waited for tow - 20 minutes - when they came it started and he drove it across to the mechanic. They once again can find nothing wrong! Any ideas??? Thanks!