Try adjusting the shift plate again. It's only the last couple of mm of the gearshift movement that actually does the engagement, the rest of the movement is syncronising/clearance.
Or it could be the front gearbox mount is loose/broken.
You don't say whether it's all the time, under acceleration or braking?
Usually jumping out of gear indicates that there are a few things worn in the 'box. Usually more common with 3rd/4th gear though.
The synchros, selector ring, gear dog teeth & selector forks can be worn and not fully engaging, the mainshaft bearing can wear or spin so that the mainshaft will move fore and aft, so when you accelerate it will jump out of gear.
And the more times it jumps out of gear, the more damage it causes.
Before you start dropping the engine and 'box out though, I would check there is nothing stopping the selector engaging fully due to the linkage.