three on the tree problem

My 66 Charger is pops out of 2nd gear when I am slowing down for traffic or just cruising down the road. The trans works fine, clutch is new and is adjusted correctly. I have heard the linkage might need adjusted. Have anyone here had the same issue, and how did you fix it? HELP PLEASE

Hello GPA,
The factory service manual will detail linkage adjustments and hopefully cure your problem but, I am inclined to think you could have a problem with the second gear synchronizer assembly. Good luck.
Mike