Yamaha fzr600 juddering at low revs, below 3500

So if you raise the revs to say about 4000 rpm and let the clutch out does it stall or judder or both will it engage at all and pull away ?

Could your clutch be sticking at lower rpms wanting to stall the bike out try adjusting clutch cable out a bit see if this helps or in as it may not be engaging all the way

If I recall correctly, the VStar engage/disengage point cannot be adjusted in terms of where in the range of the lever it is, something is not disengaged correctly. This typically indicates that there is too much freeplay . Reduce the freeplay just a tad and see if it resolves the problem.
clutch cables stretch with use and the adjustment at the clutch lever must be made on occasion. If there is no more adjustment at the lever, then follow the cable down and see if another adjustment is available. If not, you'll need to remove the cover to the clutch basket and find the adjustment there.
Now, just another thought. Bikes that have correct freeplay, but want to clunk and creep, may have sticky clutch plates/tension springs. This can happen when the oil is old or the bike has been sitting for longer intervals (previous owner?), or simply stuff accumulated and stuck to the plates. Not the most likely thing with your mileage, but don't completely rule it out.

Your clutch is not fully disengaging.Adjust the cable at the lever until you have plenty of free play.Then adjust the cable at the motor until you have full engagement then readjust lever to 3/8 in play.Fine tune at the motor.It may take a few trys but you can get it.