MacBook Pro Sound Repeatedly Crashing

The sound continually crashes on my MacBook Pro. It comes back when I restart but after a few minutes it fizzles and crackles before cutting out again.

A few days ago the whole machine ground to a halt before getting a Shockwave error message. The sound hasn't been right since.

I've removed Chrome and have reinstalled Flash, Spotify and Skype. Headphones don't seem to make any difference. Help!

Thank you.