CONVERSION OK, BUT VIDEO IS ACELERATED IF WATCH IN PC

HI, I WAS LOOKING FOR A PROGRAM THAT ALLOW ME MAKE CONVERSIONS TO WATCH HD VIDEOS IN MKV FORMAT IN XBOX 360. AFTER A LONG TIME I WAS SURPRISED WITH THIS WONDER : XENONMKV. YOU ONLY SELECT THE SOURCE VIDEO, THE TARGET DIRECTORY AND.... ENJOY, IS AAAAAAMAZING.
BUT I HAVE ONE DUDE. IF I PLAY THE MP4 VIDEO IN MY PC (ATHLON X2 5600+, 2 GB RAM, ATI HD 2600PRO) THIS IS PLAYED VERYYYY FAST, BUT SOUND GOES NORMAL. IF I PLAY THE VIDEO IN MY XBOX 360 LOOKS REALLY GOOD AND WELL SYNC WITH SOUND. MAY BE THE PROCESSOR OR THE PLAYER IN MY PC ( MEDIA PLAYER CLASSIC) ARE CAUSING THE PROBLEM?
PD: THANK'S A LOT BY THE PROGRAM : IS THE BEST TOOL FOR XBOX 360 EVER!