Lift both hands from the handlebars (not like throwing them in the air or anything, merely releasing your grip from the bars) on my CB500 while in motion, any speed really, and the front end will shake like nothing else, only a little at first, then more of a wobble before going absolutely wild, at that point I daren't let it carry on and so I control it and ride on. It's sort of like, when you see the high-sides in racing, where the rider tries to hold on but the front end shaking side to side is too violent, but obviously less violent as I know it's coming.
Any idea what's wrong? what needs fixing / replaced?
Tyre pressures good, givi box on or off this happens, stock handle bar end weights both fitted. Steering head bearing problem?