[Vwdiesel] Caddy keeps popping out of 5th gear

[Holly]

Hi all,

   I purchased a 82 VW pickup with a 5 speed. It runs great, but when I am driving at highway speed it will start popping out of 5th gear.  It stays in gear if I drive around 55 mph, but if get on a freeway at about 70 mph after a short period of time it will start popping out.   I can get it stay in gear if I hold it.  Then later after letting it sit a while it will again go okay if I run it under 55 mph.   There is no unusual noise or vibration that I notice.  Could this just be the linkage needs to be adjusted.  

Thanks

Dave