You didn't mention the isolastics. If you place the machine off the stand vertically, grab the rear wheel and pull it firmly from side to side, how much movement can you get? You have the stock head steady, right? Get a heim joint head steady or make your own if you have a workshop. What could be happening is that the engine/gearbox/swinging arm assembly is pivoting in the frame and this might only manifest at higher speeds.
The weave you mention can occur on other bikes. I had a Laverda Montjuic (500cc OHC twin) with the swinging arm mounted on rubber bushes and it weaved above 75 mph. This was a recognised problem solved by fitting bronze bushes. I only mention this because something is probably flexing more than it should.
A few things to try apart from replacing your stock head steady:
Pump up your tyres a few psi more than the pressures you usually run.
Move your weight forward.
What bars do you have? Flattish or cowhorn? This is only my opinion and I stand to be corrected on this, but I think that the weight is not sufficiently over the front wheel on a Commando with the standard, forward-facing footrests and anything other than flat bars. Try it with your feet on the pillion pegs, which will move your weight over the front end a bit more, and see what happens then. I do know from experience that moving the rider weight forward has solved speed wobbles on several bikes I have ridden, both modern and classic.
I assume your frame is straight and that your spokes are tight?
The weave you mention can occur on other bikes. I had a Laverda Montjuic (500cc OHC twin) with the swinging arm mounted on rubber bushes and it weaved above 75 mph. This was a recognised problem solved by fitting bronze bushes. I only mention this because something is probably flexing more than it should.
A few things to try apart from replacing your stock head steady:
Pump up your tyres a few psi more than the pressures you usually run.
Move your weight forward.
What bars do you have? Flattish or cowhorn? This is only my opinion and I stand to be corrected on this, but I think that the weight is not sufficiently over the front wheel on a Commando with the standard, forward-facing footrests and anything other than flat bars. Try it with your feet on the pillion pegs, which will move your weight over the front end a bit more, and see what happens then. I do know from experience that moving the rider weight forward has solved speed wobbles on several bikes I have ridden, both modern and classic.
I assume your frame is straight and that your spokes are tight?