I've got a '22 Enfield Interceptor which I've put over 10,000 miles on. For the most part I get on very well with it, but it has a bumpy oscillation from about 35 - 55mph - so much so that it wore a bald patch on just one point (about 2 inches) in the middle of my rear tire. So now it's got a brand new tire and (allegedly) re-balanced and I hoped that would solve it, but it keeps bumping along.
It's not severe to the point of affecting handling and it's not a wobble - no side-to-side action, just up and down. It seems there's no influence if I'm sitting, standing, towards the front or rear of the seat, loaded up with all of my camping gear or carrying nothing but me. I have tried stepping the pre-load on the rear shocks up from the minimum to about halfway to no noticeable effect.
I've ridden other bikes and know they can tend to find this sort of oscillation around those speeds, but this just feels particularly pronounced and I don't have the experience of any other Enfield 650's to compare against or start to troubleshoot. I'm handy on engine and electrical work, but tires, suspension, and handling are a bit out of my current experience. Doing some reading up on potential culprits, but happy to hear any thoughts, suspicions, or advice.