Worst tank slapper I had was on an RD350LC. I seriously thought I was toast. Drop the forks to flush in the top yoke. Check tyres etc as above. Check your sag. On my 09 510SMR with the stock horrible Dunlops I could hit 90mph. No hint of a wobble. On the tyre combo I'm running now - Avon PXR front and Goldspeed med rear, at 85+ it'll feel unstable. But that's cool as the bike holds the road in bends like glue. It's a trade off.
