[steven114]

Check that you're getting power, that it's the right voltage, and that it's clean. Check the datasheets to see the allowable voltage range, and make sure you're not using 5V when you should use 12, or vice versa.
One of the trickiest problems I've ever debugged electronically (I'm no expert, so this may be a 'duh' thing to anyone who knows what they're doing) was when my +5V power rail was too noisy - the multimeter registered 5 just fine, but when you hooked it up to a scope it actually oscillated between 5 and 0 at something ridiculous like 5MHz. That was playing strange games with my PIC's oscillator...

If you're sure you're getting good power, try a different one? Maybe it is broken.