chain a bit slack? one link too long, spring in the deraillleur a bit weak, or can be the freewheel is sticking a bit on the rear wheel which can lead to the top of the chain becoming very slack if you are not peadaling allowing it to jump off. Just my ideas for maybe caused by the top of the chain becoming slack. I take it that its jumping off when you are freewheeling?I have had it happen whilst pedalling normally because the front derailleur is not adjusted to go outwards far enough and is rubbing on the chain when on the smallest rear cog, but bike shop would have spotted that.