I doubt you could get it professionally & officially repaired anywhere outside of Japan. Don't know where you live, wjw, but I assume it's either in North America or Europse. Since the SNK branches in those territories are gone, you're probably out of luck for NGPC repairs.
You might want to try opening your NGPC and see if anything's loose in there. Since the screen sometimes goes blank and sometimes doesn't, my guess is that there's a wire loose.
And as Mouse_Master said, even if you could get it repaired, it would probably cost the same (or more) than just getting a new system. NGPCs still aren't that hard to find, so I'd just pick up a new one.