Is it every time the same key, or not? I personally think it is a technical problem which needs to be fixed, especially because you hadn't problems the last months.

On the Equinox I have a similar thing, when I switch off a track during playing sometimes a note hangs, but that is not a big problem, also because I do not give live performances.

No -- I think you must return it to the shop and let them check and repair the sk880, make sure you have an easy method to reproduce the problem.