Could somebody try my code with a working/original wmp & nunchuk just to check is the problem isn't software related ? That would be awesome 
Here is the arduino and processing code : http://www.woofiles.com/dl-300353-IuQvo6tw-code.rar
EDIT : After some further testing, the problem doesn't seem to come from a variable overflow, nor serial transmission, nor the processing application. I must really have a shitty sensor... damn