In case anyone wonders: what happens is that the synchromesh of the fifth gear wears for some reason, possibly insufficient lubrication, other people say that it happens if you use the fifth gear at 70km/h or below for too long. The little synchromesh teeths are grinded down more and more as km go by. I had the same symptoms (fifth gear disengaging at high load) and changed my fifth gear myself. It was clearly seen on the synchromesh teeth that they had formed an angle at the contact surface that will push the hub upwards so that the gear disengages. Later versions of the gearbox have slightly different grooves in the rods that hold the forks. These grooves are what keep the forks in engaged position. This modification was called "floating" something (dont remember the exact name right now) and it is better at keeping the gears engaged. As a result the synchromesh doesn't form this sort of angled surface even if it wears and it will last longer (until the teeths wear down completely and start breaking off I suppose). The newer revision also have slightly different synchromesh hubs. The hub for the fifth gear has an extra groove that puts additional pressure on the fifth gear downwards when engaged, holding it in place. All these modifications obviously aim at eliminating the tendency of the fifth gear to wear out and disengage.
If you want to change the fifth gear on a -89 gearbox you will need a 43/33 gear kit (43 teeth and 33 teeth on the wheels respectively). You actually only need to replace the 33 wheel and the hub, but a new 43 wheel is also included in the kit. These 43/33 kits are hard to find nowadays. There is a 45/33 fifth gear kit more available which is for a later gearbox model and has the newer hub. I fitted one of the 45/33 gears in my -89 gearbox and got a slightly higher fifth gear ratio. However I think there is a 2mm difference in height for the 33 wheel in this kit, which is bad because the position of the hub also becomes slightly out of place. For me it worked anyway for five years and 50000km but this summer if finally failed and I think it was due to this 2mm offset. It seemed to have allowed one of the 5mm steel balls to eventually pop out underneeth the synchro hub when the reverse was engaged, causing a complete lockup of the gearbox selection mechanism. That box is currently on my workshop bench and I'm looking into how to solve this with some sort of spacer. If anyone has done this and or has some other experience to share I'd be interested.
