Bendy plastic gearbox casings are not good for gears.
In order for the idler to fail as you describe, even the black ones, I would expect that there is some heat involved. It could be, as stated in the posts above, that the layshaft bearing is rubbing on the top idler. There was a fault on early casings and I don't know if it is cured yet. This will cause heat but I would have thought that you would have noticed the rough feel of the gearbox when you built it. Some people chamfer the edge of the top idler to clear the protruding bearing but if it can be shimmed over then why not? I would have thought that to add shims to the non-slipper side you would also have to remove material from the boss that locates the bearing on the slipper side of the casing, which may cause the idler to rub on that side of the casing instead.
If the layshaft bearing isn't the culprit then it may just be that your particular casing isn't holding the gears in their correct mesh. This means that you will get some heat generated. The previous failures could have permanently distorted the casing. It's a thermoplastic, so when it reaches the temperature at which it softens it will move under load and not come back. In this case, the gears will always get hot but the white POM idlers or the aluminium version, as stated in the previous posts, will probably stop it from failing.
BTW, is it the top idler that fails?
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