Most rigs have a slow-speed wobble around there. First thing is to play with air pressure. My bike is very finicky about having it spot on. next is to have the steering bearings about a quarter turn tighter than for solo use.
When all that is taken care of, different things set the slight wobble off. If it gets obnoxious, I steer left and then hard right real quick and that usually settles it down. It will get very stable at speed.
I would want to see the set-up. I think those engines are a stressed member so there's no real good way to mount a SC without a sub-frame.
The bikes are terific- especially on the freeway. Those sidecars are very light but the older ones have problems with axles breaking. This has been discussed a lot at www.sidecar.com
Whether or not it has enough power for freeway use... I don't know.