A high speed weave is very very common on 600cc dual sport bikes.
Mostly they do it right from new. Change of tires might help but I doubt it.
Greg has pointed you to a few things to check, although I doubt head bearings will cure this problem.
I've had the exact weave you describe, at the same speed on the following bikes.
XL600 Honda, (near new)
KLR650 (when brand new)
XR650-L (from new)
DR650 (brand new, 2005 model....weave started at 90mph)
Various things that might help:
1. relax your grip on the bars at speed
2. crank up rear pre-load a bit
3. raise fork tube up in triple clamps about half inch (lowers front of bike)
4. Fit a steering damper.
5. Snug up Steering head bearing...but only slightly.
As mentioned, the front fender contributes some to this but is NOT the sole
cause of high speed weave. Sometimes fitting a mini sheild can help too.
Fiddle around some, don't worry. Nothing wrong with bike.