Sounds like bad (dirty) contacts that may need to be cleaned...or replaced. Did you try canned air spraying under the key bed?

If you decide to take it apart:
http://www.psrtutorial.com/music/projects/ContactStrips.html

If it were me, I'd contact the seller and if you used Paypal/Ebay you should have some protection.

I can understand your frustration
Good Luck