my 2004 mazda 6 2.0td has started to 'jump' out of fifth gear afier approx 20 - 30 miles..
is there anything that would cause this to happen or is it more likely to be an internal fault

its done 42000miles
fully serviced
its a manual gearbox

any help much appreciated

 

When manual transmissions kick out of a gear, it's almost invariably because it wasn't properly in that gear to start with. Causes are worn/ incorrectly set engine mounts, worn bushings on the shift lever, and incorrect setup of the transmission linkage.
It is very rare for it to be due to something inside the gearbox itself on a car which has been properly looked after.

 

On the other hand, I once owned an MG. One day I missed a 3rd gear shift. From that day on it ground into, and popped out of third gear unless held in place.
What I'm getting at is that it could also be a worn or damaged synchronizer.

 

I believe the 2006-2007 Civic Si's also were having problems with this same sort of thing. I believe it was mostly third gear on these vehicles however.

 

The Mazda 5-sps have single syncroed gears after 2nd. My best guess is that the Hondas do too. It's not all that uncommon.