Not sure if anyone can attempt to tell me what the issue is or possible cost...I have an automatic with 77k mi.

About two weeks ago, I was driving on the interstate at about 80 MPH. I started hearing a whistling sound in the engine. When slowing to get off at an exit ramp, the car started acting like it was having trouble going back down in gear. Once it did I had to put my foot all the way down on the gas pedal to go, but the RPMs shot up and the car would only go about 20 MPH and stayed there for a good 2 minutes then got up to about 45 MPH. Once I got home, I restarted it drove it around the block testing different speeds and it was fine. Drove it for a week, it was fine. Then over the weekend it did the similar to this but I was at a red light coming to a stop and it jolted a bit and once at at complete stop the car even with the foot off the break didn't move, kinda acted like it was in neutral. I restarted and it worked fine. Drove for another week, did the same exact thing at a turn lane coming to a stop but this time it completely shut off. I restarted and worked fine. I have never heard the whistling sound again since the first initial time it has done this. My mechanic cannot see the car until next week. I don't take it to the dealer because they charge way to much. But wondering if if anyone else had had a problem like this or knows about the vehicle enough to possibly know what is causing this. I have pegged it to be doing this about every 20 to 25 restarts. Also, no warning indicators are coming on either. Thanks for any help!!