If its not the shift cables, you will need to replace one or multiple parts. Everytime it pops out of gear, the slider and 3rd gear are wearing out the dog teeth.

Until you open up the trans and see how much damage there is. typically you will have to replace the slider, and 3rd gear. IF the dogs (ball detent assy) is worn you will have to replace those too.

I have had 2 trannys with that issue. One was 5th gear and the dog teeth were rounded over so bad it would pop out immediatly. The other trans which still remains in my talon because i am too lazy to pull it only pops out of gear on occasion. Both tranny's will require a complete new gear assy. They only take me about 30mins to replace them, not hard if you know what you are doing.

The Dog teeth are shaped like a trapaziod so that the slider teeth when interlocked, holds the gear/slider together. If that trapazoidal shape wears, the two pieces can slide apart. The "dogs" or the ball detent assy's are what hold the lider from shifting out of gear as you lift your foot on and off the gas in the same gear.

If you can find someone near you to split the cases and disassemble the trans, i have the parts for you. That is assuming the shift cables and bushing are not the problem.

Terry