Originally Posted by Kev

First thing I would do is centralise the front wheel, otherwise the bike will track & cause a weave.

I am not sure how old your bike is or how many miles it has done, so here are a few suggestions to look at.

Check the rear wheel alignment, meaning the chain adjusters are equal.

Are the steering head bearings ok & adjust correctly not to tight or to loose.

What make of tyres are you running & what are their sizes, what is the tyre pressures set at.

What is your rear spring set to, which notch & what is your approx weight.

Also check the swinging arm bearings for play.

Check the front & rear wheel bearings for free play.