Car jumps out of gear

[Dick Carne]

On our recently restored sedan, I am finding that the car will often jump out of high gear when I have let off the gas and it is in a continual coasting pattern (usually downhill). All of the transmission gears, bearings, shafts, seals, etc. were replaced; in the shifter housing, the detent springs and posts were replaced, the forks inspected and flipped; and the flywheel housing was properly shimmed against the back of the engine block. I am at somewhat of a loss in terms of what to do next. Any suggestions and guidance is most appreciated. Thanks guys.

Dick