'09 Voyager running rough at speed

[MrMikey]

2009 Voyager, 53,000kms. starts fine, idles fine, runs around 1200± rpm when cold and then drops to around 900± rpm after a couple of minutes.
Seems to pull ok but when running steady speed around 100KPH at 3000 rpm in 5th there seems to be a lot of engine vibration, more so than a twin should have.
If I pull the clutch at 100kph so the engine drops to idle, the vibration stops so it's not drive train, least ways not from the rear wheel to the output shaft, at least in my thoughts.
I'm thinking an air leak would show itself at idle as well, I'm going to use some WD40 and spray around any intake joint and see if anything changes.
Going to change the plugs even tho it sounds too simple a solution.

Anyone got any other thoughts?......Mike