My 65 Vette shifts perfectly. But downshift with some difficulty. This is an intermittent issue. Sometimes it down shift fine. But often needs extra force. Every gear will shift fine sometimes but 4th--->3rd or 3rd ---->2nd or 2nd ---> 1st is taking more pressure and occassionally will bulk and not go into the lower gear.

Anybody know how to diagnose this?

 

Welcome to the forum.
You probably will need to adjust the shifter linkage. The procedure is described in the factory service manual. Maybe YouTube..