OK folks, you're all on the right track, but some additional information may be useful:
A GFCI outlet can be purchased for about $7.00 and a GFCI circuit breaker will cost $30.00+ depending on the brand of service panel you have. Please note however that these devices are not meant to protect equipment; only people. They work by detecting leakage of current between the hot side of a circuit and ground. Then they shut down the circuit before you can even feel a tickle. They won't protect against over-voltage situations (lightning) or dead shorts (Damn! I twisted those two wires together!*). In order for a GFCI to work properly, the circuit needs to be grounded. That is, there needs to be a third (ground) wire running from the service panel to the outlet box. I have seen many instances where this is not the case. A lot of older houses are wired without a separate ground wire, and I've never seen a case where a GFCI will work without it.
Since reversing your plug seemed to resolve the problem, my guess is that your outlet may be wired backward. Turn off the power at your service panel and remove the outlet from the wall. The black (hot) wire should be connected to the brass screw, and the white (common) to the silver screw. If not, carefully trade them. Also, a third (ground) wire should be in there. It will either be bare copper or green jacketed. This should be connected to the ground lug on the outlet. Note that just because you have a three prong outlet, it is not necessarily "grounded". Most hardware stores sell a plug-in device that will test your outlets for proper wiring polarity and ground. It might be a good investment in your case.
As for everyone else: If you spent any money at all on your musical equipment, it is worth the cost of an electrician to install that missing ground wire if you don't have it. Also, invest in surge protection just like you do for your personal computer. You'll thank yourself many times over when the unexpected happens.
*Most people assume that circuit breakers protect against equipment fires. Not so! Breakers are there to protect the wiring in your house. A coffee maker or wall powerpak can melt into a glob of plastic and never draw the 20+ amps required to trip a breaker.