RMak is bang on the money with the link he has provided you with (for the majority), as this was my problem too some time ago.

You may need to look at the contact faces on the cluster a bit more closely. I thought the emery paper trick would solve what sounds like the exact same problem as you're having, but it didn't. After searching I found that excellent post by Quiksilver and since dropping a bit of solder onto the burnt out contact face over 18 months ago, I have had no issues whatsoever.

Good luck.