It has all the hallmarks of an intermittent connection - perhaps triggered by fluctuations in temperature.
The kind of thing that's almost impossible to find and fix, unfortunately. And being 3 years old, it won't be under warranty any more.
Could you install a PC card sound card that has a mic. socket?
e.g.
Creative Labs Online Store (UK) | Sound Blaster | Buy Creative Sound Blaster X-Fi Notebook
Not a cheap fix, but if it avoids all the hassle - and gives decent sound, as a bonus - it might be worth it.
SD