Hmmm, that sounds like it could be the hub. Is it particularly bad in any of the gears? SA don't seem to have an exploded diagram of the S3X on their website like they do with most of their hubs, so all I know about its innards is gleaned from my imperfect knowledge of the 5-speed it's apparently similar to. As such, I won't make any judgements just yet.

How much play is there in the rear wheel?

Pkshields may be onto something, high gears tend to lead to a lot of standing on the pedals, which puts cyclical stresses on the drivetrain and rear wheel, as opposed to the more constant stress from staying seated and spinning. What size sprockets are you using front and rear?

Also, is there any slack in your chain? SA hubs can have issues if the chain is too tight.