MPU-9150 After many hours data all 0xFF & can't be recovered

MPU-9150
I2C at 400khz
Polling at a rate of 500Hz

The Accel, Gyro and magnetometer data are all believable and all is generally well.
After operating for a number of hours (generally overnight) the device will report 0xFF in all its registers and will not respond to configuration or initialisation.
Strangely the I2C continues to work perfectly generating all the necessary ACKs etc in response to my queries, but with 0xFF for data.
It requires a power cycle to recover the situation as nothing I can do on the I2C has any effect.

Has anyone else suffered this problem and found the answer to what I'm doing wrong?

Thanks
George