I've never had this problem in my other manual cars. well, lately when i'm downshifting at high speeds, I can't even get into 3rd or 4th. I hear a VERY loud grind and wont go in. I have to put it back into 5th. Not sure if its the syncros, its too loud. All I have is ACT HD with 60K miles on it (84000K overall)

 

Try adjusting the clutch, change the transmission fluid Redline or BG Synchro. If all above fails, time for a new clutch.