i’ve had this problem that has been recurring in my 92 gs-r… my car comes out of 3rd gear whenever i turn the wheel left. it can be a smooth left, or a sharp one… travelling speed does not seem to be a factor. and this happens ONLY in 3rd and going through a left. i can go hard through a right (like a ring-road) and nothing happens… any ideas? i haven’t had a chance to lift the car up and have a look yet, though. is gay.
I’ve had a similar problem, as far as I can tell the problem is worn synchros. 
Sometimes when a motor mount is extremely broken in certain situations the engine will move enough to pull the tranyn away from the shifter and pop it out of gear. I guess just check your mounts quick anyways.