Why does my R button press automatically?

Here's the deal: my SNES presses the R button on its own. Not repeatedly, but Mega Man X, for example, switches weapons every 30 seconds or so, Castlevania 4 keeps throwing knifes, and Super Metroid looks around. It really gets on my nerves, and I've tried both of my controllers. It makes me not want to play my SNES. Any help?

Thanks!

Blake