1992 Olds 88 Royale won't shift into gears correctly

I have had my transmission flushed and filter changed, but I am still having a problem with my gears going into reverse or drive...it seems to happen after my car sits for a couple of hours...the only way to get it to seem to shift into gear is to shut off the engine and try again and if that works keep trying until it does work...this has been going on for about a week or so now....yes, it does have 218,+ thousand miles, but it's a charm of a car....any suggestions or comments?