It could well be the squared off rear. The condition/shape of the rear tyre can do funny things to the front end. Probably worth replacing anyway - see if it makes a difference.

Double check your headrace bearings aren't loose, tight or notchy. This is quite a common cause for wobbliness at speed.

Once you've ruled out the rear tyre and headraces, then have a look at fiddling with tyre pressures and also check the suspension sag/ride height is correct.

__________________