Originally posted by jlicrx
try adjusting your clutch - high rpm grinding is usually not caused by bad synchros but will sure make them bad - if clutch is not adjusted right, you won't have enough air gap when the clutch is disengaged and due to cetrifugal force the disc will remain in contact with the flywheel - like trying to shift without the clutch - if synchros are bad it will usually jump out of gear.
Would a misadjusted clutch only affect one gear though? I never grind any other gear except 3rd at high RPM.