Speakers Keep Losing Bass! Help!

Hey, I have Elitech Super Woofer speakers for my computer... I've had them for about two years now... and now every so often the bass in the speakers would just go away and come back later... it doesn't make sense, sometimes I'll be listening to a song and then I hear this popping sound and the bass is gone, but the speakers still work... then later I hear another popping sound and the bass is back... is there anyway to fix this? or are my speakers just going bad? Thanks!