Sounds like your box has decided to munch itself like my box just did
Bit of background on my car/box...
Car has 115,000klms on the clock, of which about 90,000klms has been over 190rwkw.
Car gets driven hard all the time, goes on regular trackdays and too the drags every now and then. So getting about 115,000klms out it is pretty good i reckon!!!!
For about the last 1-2 years the car has been popping out of 4th gear when under load, and it gets a bit bumpy. On the track it was extremely annoying and when on a spirited drive through some twisties, it got a bit dangerous when it would pop out of gear.
It was just an annoying little thing, untill on day it started to make a terrible rattling noise in every gear. Sounded like just a really really really loose box. I had the box ripped out, and inspected and what had happened was the under the 4th gear circlip is a washer which the cirlip holds in place. This apparently moved and the shaft (layshaft or counter shaft... can't remember which one) decided to move. This explained the excessive movement of the gear stick when giving it a bootfull and why it popped out of 4th gear when under load. Now with the shaft slightly off center, it wore about 5-6mm off the teeth of 1 gear, i think its 4th gear they said, and damaged 1st, 2nd, 3rd, and the reverse idlergear. Also damaged some bearings and synchros.
I am rebuilding the box back to stock so that i know it will be 100% and it will buy me a couple of months while i save for a full Nismo 6 speed gearbox or a full HPI 6 speed gearbox, the HPI one is my preference.