Microcontroller writes additional data on high-frequent writes
The way to diagnose these sorts of problems is to cut them in half. Whose fault is this? The C-Control or the PC? If we can look at the actual data sent on the wire, we can tell whereabouts the fault lies.

Start by sending a simple, easily identifiable character, over and over again. I like to use the one used as an example on the Wikipedia's RS232 page because you can easily see what the waveform is supposed to look like.



Now you'll need an oscilloscope. If you can't afford one, I highly recommend the Saleae Logic analyser. It's not that expensive, and it will save you literally days off your life. Look at the waveform on the bus, and check that it matches the waveform you expct. Check for any rogue characters transmitted afterwards.

Assuming everything looks OK, then the problem is somewhere in the PC.  If the rogue 119 characters appear, then you know the problem is in the C-Control