i got the same problem last month... mine is quite a serious one, one of the component broken i ended up overhault the whole gear box... but initially, they suspect just a wore out contact patch between the gears and required a repair kit, which will cost about RM3k including labour.....
another possible cause mentioned by jarance is the contact positioner.... few hundreds to get it done....
or maybe just the solonoid has gone....
have to get ur mechanic to do a computer check first....