<VV> Pops out of Third!!! ARGH!!!!

[Jeff Wilson]

Well, well, well- drove my new (to me) 65 3-spd to work today 
for the first time. Part of the route is 6/ 8 miles is on the highway 
at the usual 55/ 60 MPH. 

Big surprise! At about 60, the tranny 'pops out' of third gear, slips 
into neutral! It pops back into gear easily if I press the clutch in, 
but a mile or so later- POP!

Holding my hand on shifter ball seems to help- but I have felt it
'push' out of gear while I hold it.

Problem get worse as speed increases over 55. Seems to hold fine 
at speeds up to 55- is iffy at 60, but won't hold for s**t at 65.

This is real problem, as I want this to be my daily work-commuter
car!

Please tell me this is just a linkage adjustment....

CJ Wilson