I agree with the earlier comment about the 'reconditioned' gearbox. Would be worth taking the side cover off - takes about 2 minutes - and inspecting the gears. Look carefully at the little 'dog' teeth on 2nd gear - and compare them with the ones on 4th gear. 4th gear 'never' wears - so the teeth will be nice and sharp - and that's also what the teeth on 2nd gear should look like! If however they are rounded and worn looking - then obviously a new 2nd gear hasn't been fitted, and it may crunch/jump out of gear.[frame]

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In this picture - 2nd gear is to the left - and the teeth are pretty much as good as the 4th gear (and 3rd gear) teeth.[frame]

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In this second picture - the 2nd gear teeth are noticeably more worn (as are 3rd and 4th to a lesser extent) . It's probably still a serviceable gearbox, but obviously not as good as the first one !
It's difficult to see 1st gear - or the laygear - but with a good bright light and some squinting - you should at least be able to see if there are any teeth missing (or seriously chipped) on the laygear where it meets up with first/reverse gear. Hopefully not!