high idling gear lock problem

Whenever I go through a drivethru or have to let the car idle for a
long time (train or red light or slow traffic) I find that the car
starts revving really high. A couple of times i have paniced and
shoved the car into park and then the gear thingy locks and I have to
take the key out and wait about 5-10 minutes before I can unlock it,
sometimes its better but lately it just keeps revving so high it
sounds like its just going to blow up.

It seems to be getting worse and it seems to happen only when it is
hot out....I don’t know if it has a relation to the a/c or just
because its hot but sometimes when I have the a/c it doesn’t happen.

I notice that if I go through a drive thru its like gas pools while Im
idling or something because after I leave the drive thru the car will
idle for the next 15 minutes at 70 kph and its very difficult to
brake.

Anyone have any ideas before I take it in as to what this could be so
I can prepare myself ahead of time if I need a new transmission...

 

make? model? age? mileage?