I have experienced a similar problem. I think the PC receives something that the uC has not explicitly sent, probably the PC interprets as a start bit some transient in its RxD line. I wonder who generates this transient, maybe the MAX232 at power up or the uC when it is reset.
I have solved the problem clearing the PC receiving buffer (I work in DOS), something similar to reopen the stream (in Windows) any time I begin a new link.
When this is not possible, I identify the packets with a header, so that all the packets beginning with FFh are rejected.

I'd like to know the real cause of this behaviour too!.

Alfredo.