On Alfa number 15
AO Platinum Member
Join Date: Dec 2005
Location: United Kingdom
Re: 3rd gear slipping out....
I had the same problem on a Rover, it turned out to be the countershaft support bearing in the gearbox had broken up. I was luck as all I had to do was remove the casing, take it off the shaft and re-fit a new one.
Other things to check are the linkage, and the engine/gerabox mounts. If the mounts are broken or losse it may be moving around enough to pull against the linkage and pop it out of gear.
I would doubt it was a syncromesh as they usually cause crunching when going into gear when they fail.