All,
I have definitely seen this problem in Windows (7). I found that it was a power problem. I my Arduino had a heavy power draw (an LED strip in my case) attached to it, the serial connection (COMt6 in my case) was very, very flaky. Detaching the LED strip made the serial connection very stable.
I was able to keep the LED strip attached and keep the serial connection stable by adding an external 5V power supply to my Arduino.