Shifter stuck on park in my 2000 S500 !!!!!!!!

My shifter in my 2000 S500 is stuck in park - only in the morning for the pat few days - I had to yank it Yesterday to move it to R , after that it operated 100% perfect all day . Today I had to do the same thing too ! it operated normally all day except if the car has been parked more than 4-5 hours - What could the problem be . Please help!