[Alpines] 3rd Gear pop out

[WSuman at aol.com]

On my SV, ever since I'd owned it, I'd been having an issue  with the 
tranny jumping out of third gear when I came off throttle.
 
I had a spare tranny rebuilt, new synchros, etc.  It was fine for  1000 
miles or so, but now it too pops out of third when I unload it.
 
If I stay on throttle, it stays in gear.  Could it be a linkage  issue?  Is 
this inherent to them?
 
Many thanks,
Walt