So how many miles on the BB? Then what quality?
I normally use "XT" shimano bottom brackets and only ever change them on the Tandem- where they do take a lot of Punishment. But I have had XT BB's fail on me after what I think is a relatively short time- and then I have a Mountain bike that has had the same BB in it for about 20,000 miles.
I doubt that it is your riding style- Unless you are a clyde mashing up steep hills and then you would knock them out relatively quickly. Just go for LX or XT quality in the Shimano range and you should be OK.
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How long was I in the army? Five foot seven.
Spike Milligan