NO SPEAKER AUDIO iPHONE 3G

About 3 months ago I noticed that my iPhone 3G had intermittent sound coming from the speakers in the phone - like there way a short in the wiring. The phone has NEVER been dropped or abused in any way! Finally, the spkrs just stopped working all together! The headphone jack worked, but no speakers! Cant hear the phone ring, cant play the iPod - nothing. A couple of days ago, they suddenly came back on!! When the spkrs came back on I shook the phone and tapped it on on a solid surface (while in the iPod play mode) and it played just fine! Then, just as quickly, the spkrs died COMPLETELY again.
Does anyone have a clue what might be causing this?? Is it a software issue or a hardware issue? Its really annoying! Im about toe switch to Android!! Thanks
Iphone 3G iPhone OS 3.1.3