Hi Tony,
I remember a guy on another forum with an xj600 divi, did 80,000-odd miles using it every day with no major problems.
The UK xj600 divi had a center stand as original equipment.......now, if you are letting it warm up on the sidestand, there can be oil starvation issues on the right side of the engine.
Not sure about lowering the suspension, I'm 6 foot, so my xj6 always seemed small to me.....
My best advice.....pull the air filter out, check for "mayonnaise" (that's what it looks like) in the air box. Lots of causes, but an oil change and cleaning out the crankcase air breather pipe (can't remember exactly where it is) will often rectify the problem.
Good luck.