I'd take the cluster out to inspect what's going on. A lot of times, the pins on the board that go into the harness connector are loose or broken.

You can either solder the pins to the board (traditional route) or a friend of mine came up with an even better idea was to use tiny machine screws in the backside of the pin. That stretches the pin out and makes it much tighter in the board but also gives you that solid mechanical lock.

I'm now thinking a combination of screws & solder would make it so you'll never have to pull a cluster again!

Oh, and I also second the vote for adding a secondary ground wire/strap.