I've read that this can be a Sprint issue as much as a Hero issue. You might try updating your PRL (settings->about phone->system updates->update prl); somebody said it did help a bit but I think it only helps if you are roaming a lot. Your PRL is the Preferred Roaming List, and is the list of carriers your phone will choose when it is roaming. Somebody else said that if your current Sprint tower is overcrowded it will flip you over to roaming behind the scenes but I'm not sure how accurate that is.
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