Chain drops

bikeit65

i have a triple nine speed set up on my bike, when i select the middle chain ring at the front and the first or second cassette at the back and dont use the bike for a while and go to move the bike backwards the chain will not run smooth and drops down to the next cassette, the same thing happens if for some reason i peddle backwards.
I replaced the cassette middle chainring and chain about three months ago but this problem has only started recently.
Any help please