They're normally totally reliable. I run several for all sorts of things; one runs cgminer and hasn't rebooted for months.

Two things that maybe causing problems (and I've experienced both of these):

- You power supply may be inadequate (make sure it can supply more than 1A). Try swapping your current one out and see how it goes.

- The pi may be running out of swap space if the program has high memory requirements. (I don't get a problem with cgminer and the default swap space, but I don't know what bfgminer needs). You can google for information on how to change this if necessary.