I went on a long 5 hour trip and on the way there I only got 40 minutes of logging data before it shut itself off

On the way home of the same trip it logged 52 minutes....

I am logging to a SD card, it's a 2 gig brand name card. I have never had any problems with it, but, this is the first time I have tried to log anything over an hour.

The low power shut off is at 0 - every value in the flash scan (other than the white back ground blue letters) is completely stock.

Any ideas???