I had the same problem in my X3. There is a rubber band along the whole keyboard with two conductive lines which 'touch' two contacts on a board for every key you press. Probably, the conductive covering is worn out or dirty.

I opened my X3, took all the screws from the one-sided PCB which has the contacts and I then could clean the conductive lines of the rubber with alcohol (if you rub too much I'm afraid you'll remove the covering). It worked OK for some time, but in the end I had to change it. I ordered a new rubber band from my Korg dealer and changed it (it costed about 5000 pesetas, $35 if I am right).

You could alternatively try to 'rotate' 180 degrees the band, and it could be that the worn-out zones do not fall under the key pressing points.

From what I've seen from the N364 & N264 their mechanics shouldn't differ a bit from the X2 & X3.

I hope it helps.