My car has been having three issues in the past.

My car has been having three issues in the past week; the engine powers down and the speed slows once the ac is switched on. Secondly, the engine goes off when driving slowly or in traffic. I have taken it the workshop where there was an attempt to scan it to detect the faults but the scanner couldn't detect the car system. It's been a guessing game when it comes to identifying the real problem. Lastly, I noticed, the gear is getting stiff (its an automatic car). Its becoming hard to switch from park-drive-reverse. Please what should my mechanic be looking out for? Thank you