It sounds like drift could be an issue here. First try Intersense's calibration procedures for reducing drift. Download ISDemo from Intersense if you don't have it installed on your computer yet. Go to the
InertiaCube downloads page and download the InertiaCube Product CD. ISDemo can be found in the programs folder. Instructions for the calibration procedures can be found in their documentation. Keep the IC4 mounted on the ST50 for the calibration since they are used together. You will need to perform both "Fixed Metal Calibration" and "Magnetic Environment".
If you still have drift issues after calibrating, you can use vizconnect's optical heading plug-in with two position sensors mounted on either side of the HMD to correct it.