In that case I'd say the rear end riding too low, or you're running too much sag at the back.

Try adding preload first, as that's the easiest thing to alter in small steps. Then change the linkage bones back and try again. The most important thing is to do is one thing at a time. Also, test out the changes in the same corner and at the same speed if at all possible. Limit your variables as far as you can.

It's also worth checking your forks aren't topping out under acceleration as well (due to the back end riding low). If they top out during a corner and the front drops into a dip it will/could initiate a waggle.

I'm assuming you managed to arrest the tank slapper by letting off the gas, shifting the weight forward onto the front? If that's the case, then that's further evidence to support the low rear/high front scenario...

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