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From what you say, it is probably worth replacing the shift components, particularly the stopper arm and spring which are known to give trouble. It is very difficult to tell whether the spring is weak or not as no specification for it are given to test against. None of these parts are expensive and are easy to replace (especially as you've already got the clutch out). It is worth giving this a go first, before splitting the motor. Always best to try one thing at a time, starting with the cheapest and easiest!

We have seen several instances where failed stopper arm components have caused two gears to select at the same time resulting in transmission lock-up and serious engine damage.

Just out of interest, does it jump out of third into another gear, or into a "false neutral"?

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3rd tends to be the gear most commonly shifted into and out of (being in the middle, so tends to suffer the most wear on the pinion dogs over time. Any slackness in the selector mechanism (shaft, stopper arm, forks etc) will highlight any wear on the dogs or the slots they engage with. 3rd does seem to be the gear that, over the years, has given rise to the most trouble in terms of selection and reliability on the 660 motor.

In the past, if gear selection becomes tricky or jumps out, swift action to refurbish the shift shaft components has cured the problems and prevented further damage. We have seen several 3rd gears break (and ruin engines) due to stopper lever arm/bearing failure and continued use with a sloppy shift mechanism.

If repair of the shift shaft mechanism does not cure the problem, then splitting the crankcase and examination of 3rd gear pinion is probably going to be necessary.

Let's hope it is the shift shaft is the source of your woes...

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