I got it to work now, after i decided to throw out all this ringbuffer stuff since i found a very simple command to read serial messages from the arduino on this forum (Serial_IsCharReceived() and Serial_RxByte()).
The parsing i do on the function channelMidiMessage, i save the messages in an array of length 3 and as soon as i have 3 messages i put it together and send it further on.
If anyone is interested, my colleague will put the project on sourceforge as soon as were finished with it and i will post the link here 
Thx for the help