My friend has a 99 LS1 M6. The car shifts fine from 4-5-6, but when you try to downshift from 6-5 it wont go into 5th. Shifting from 6-4 then back to 5 works fine too. This happens at any rpm. Anyone else have this problem?

 

I had the same exact problem on my M6. Under warranty GM relplaced the fork and syncro. I Have since gone through the entire trans with D&D hardware. The trans is much better!
I also installed a B&M Ripper