2001 engine shuts down

My 2001 sub engine shuts down,once while backing up once while pulling out of the service station, but it always starts back up.It has yet to shut down while on the highway.Can anyone tell me if there is a Throttle Position Sensor that would cause that.Also in the summertime
when I would turn the a/c on sometimes the engine tach would fluctuate
from mormal to below idle.Then when you would take off at highway speed it would correct itself.

I would appericate any help anybody would give me.

thanks
sub2001