Handlebar bucking

Could you elaborate on "bucking".

Do you mean the front-end bounces up and down? Or do you mean the handlebars turn rapidly side to side - oscillate?

If the later, this is fairly typical Commando behaviour at around 40mph. Indeed, it's typical of a lot of bikes of the period. But it can be dialled out, or at least down.

Do you have the front and rear wheel tyre sizes as 4:10 / 19 at both ends?  Some Commando owners try to opt for originality but the bike definitely handles better with a 3:60 / 19 at the front.  As Stan Thomas has elaborated above, there can be a tendency for most of the larger motorcycles of the period to wobble a little at that speed [see early BMW 75/5 models for details].  It's very noticeable compared to a model with a Featherbed frame.  You haven't mentioned what the handling is like when cornering or the type of handlebars that you have fitted or whether you have had the wheels balanced, which is especially important for the front wheel.  Pop the disc brake calliper off and spin the wheel to see where the front wheel stops - if it stops in the same place each time it is probably out of balance.  You can buy wheel-weights or use sections of heavy solder wound around the spoke-nipples. 
Good luck!

oval drum?.

Loose head bearings?  Or no oil in the forks?

I am going to put my money on the front wheel not being balanced. This could be down to a buckled rim, poor spoking or an unbalanced tyre.

If the frequency increases with road speed then, as Philip says, front wheel/tyre out of balance / out of round / bent.

You don't say if it disappears as you speed up. I had a steering head oscillation develop at around 40 - 50 mph on my Mk 3 850 which went away once over that speed. Cured it by fitting a steering damper. Funnily enough after bike had a total professional rebuild the oscillation disappeared so no need for damper anymore. If your "bouncing" or whatever steering issue it is gets worse with speed then likely it's front wheel balance which is simple to cure.

Check that the front tyre is fully blown out on the wheel rim.  There is usually a line around the tyre where you can see if it is the same distance from the wheel rim all around the circumference.  You need to check both sides.  If not, deflate the tyre, apply lubricant all round between tyre and rim, then reinflate until it is even.  If necessary, take the pressure well above manufacturers' recommended pressure until the line is even and then let it back to the correct pressure.

Bet its out of balance - I had something similar years ago after fitting a new front tyre.

Regards - Paul.

Have you checked the front brake shoes? Perhaps a return spring has broken or a brake liner is slack and at low wheel rpm is self servoing on and off.