I have had terrible undulating vibrations that came and went whenever I rode my malvern star. Didn't do it in first gear and only occurred under high load situations like any time the gradient was up hill even slightly. It felt at times like riding a horse and the goose neck would bob up and down rapidly with a groong groong groong noise. Sometimes it would stop if I closed and reopened the throttle. I initially thought it must be something wrong with one of the two chains or something else in the jackshaft assembly. One day whilst rotating the chains with the clutch locked in, I noticed a slight side to side wobble in the engine sprocket's gear teeth as well as a slight up and down(out of round) movement in the round part of the sprocket. I found one on ebay with free shipping for under $10. When it eventually came it had a narrower key slot than the wobbly one - I converted it to a square key width with a hack saw.
Whilst the hole in the wobbly one may have been only slightly crooked, changing it for a straight one has made a dramatic difference- the bobbing front end thing hasn't happened since!!
This engine which I bought from ozbicycleengines website also had a wobbly clutch/bevel wheel which I had to replace, at least I recently noticed when you search for bike engine kits on ebay(au) you have to go to the very last page and scroll down to the very last item right down the bottom to find their engine kit!
