Sounds like you may have a pcb fault however I suggest you get onto the Roland website and download one of their factory manuals cos it sounds like you don't have one. You also don't say whether or not it's new or second hand however the following is some food for thought:
I do the sound engineering for various bands and have come across something similar. My partner plays keys in her band and has some very high end Roland and Korg boards - one of which went into the shop for a similar fault ie keys which weren't physically sticky but either didn't respond or stayed on!!
Before you take it in for repair (costly), you should check out:
1/ nothing has been spilled on it - obvious but worth checking.
2/ you have checked out the ''attack'' setting on the key board
3/ the board has not been crushed/dropped when you've been at gigs - can cause loose solder joints on the pcbs - especially if still warm
4/ You have tried a factory reset - sounds obvious but usually clears 99% of faults - bummer if you have some presets saved cos you'll lose the lot.
5/ Check different programs and see if the dull keys are still inactive
6/ Can't see how the power supply is causing problems with specific keys - either the external 9v unit (1200ma) works or it doesn't. Roland website will also specify the adapter you should be using although they seem to use the same unit for just about everything. They aren't too expensive and if you gig the board regularly you should always have a spare one with you anyway.
If none of this is of any use I'm afraid you may have to make the choice of paying for an 'expert' to look at it or scrapping it if the cost is too high.
Hope this gives you something else to think about before spending your hard earned......
