um,....... lets see which generic advice question i can ask here - I NEED MORE SPECS! just kidding.
Why do you think I always ask that.
Got all of your windows updates and the absolute atest drivers for your sound card?
I can think of reason why it might happen, but not ones that would happen sometimes and be fixed by a restart.
When it occurs have you tried using other mic programs such as HL talk or even windows voice recorder to see if it is actually vent or your sound device / card.