Here's something to check without removing the transmission. On DSM transmissions, the bearings and shafts are basically held tight by 2 36mm nuts. As the tranny gets older and wears down, excess slack develops and the nuts can have a tendancy to back off. The nuts will literally grind thru the out case in worst case scenarios.
Remove the passenger wheel, remove splash guard, drain transmission, remove the outer 14mm head bolts for the outer cover. Remove the cover and be sure to watch for the reverse brake synchro as it may fall out, not to worry as it is only held in place by a spring washer on the outer cover. Once the cover is removed, you'll easily see the 2 nuts that i'm talking about. If there is any play in them, you need to tighten them up. The spec is something like 120ft lbs. Once you get it tight, re-dimple the nuts.
Then reinstall everything (put some oil on the reverse brake synchro and put it in the 5th/rev assembly, you may have to turn the synchro until it falls into place. When reinstalling the outer cover, use non-hardening gasket sealant from a parts store as that is nearly identical to the OE stuff. Hondabond also works well.
I have a spare transmission lying here that I can take pics if needed to help.
This was known as the 'shifter walk' fix back in the day. When in 5th gear and gas was applied the shift knob would 'walk' in the shift base. And when it got bad enough would actually pop out of gear when gas was applied.
Now be aware that if the nuts are pretty loose or have backed off into the case, it's usually a sign that the transmission is getting pretty wore out, but nonetheless it can extend it's life considerably and buy you some time.
Wiz