Is my Arduino datalogger overheating?

Hi everyone,

A few months back, I put together an Arduino Uno and datalogging shield to making a simple temperature/solar irradiance logger.

The logger setup is powered by 4 D-cell batteries takes recordings every 30 minutes on an SD card. I packaged everything in a box, and it's now installed overseas (Vietnam):

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For the last 2 months, it's been working exactly as intended, taking measurements once every half hour as it is programmed and tested.

Lately, the guys down at site moved the box to a new spot and something has gone wrong. From 8 pm to 9 am, the logger is taking half-hour recordings as it should, then it around 10 or 11 am, the logger stops taking recordings, then it starts recording again at 8 pm. So I'm missing 8-9 hours of recordings every day (between 10 am and 8 pm).

Here's a snippet from the excel file on the sd card:

Datetime Irradiance Temperature "2015/5/12 7:27:8" 0 27.19 "2015/5/12 7:58:18" 1.22 27.37 "2015/5/12 8:29:27" 34.18 27.62 "2015/5/12 9:0:37" 39.06 28.06 "2015/5/12 9:31:48" 114.75 29 "2015/5/12 10:34:17" 177 30.19 Jumps 9 hours "2015/5/12 19:59:22" 7.32 31.62 "2015/5/12 20:30:37" 20.7 31.37 "2015/5/12 21:1:50" 35.4 31.12 "2015/5/12 21:33:4" 0 30 "2015/5/12 22:4:16" 0 29.75 "2015/5/12 22:35:27" 0 28.81

You notice in this table that the Arduino cuts out at 10:30 am, when the temperature is 30 C and it comes back on at 8 pm when the temperature is HIGHER..this made me rule out overheating. However, the temperature probe is actually sticking outside the box; I'm not measuring temperature inside the box, which probably goes up a bit when sunlight is hitting the box. Could this explain the gap in my data? Overheating during the daylight hours causing the Arduino/logger shield to "cut out"?

As a first step, we tried turning the Arduino on and off and formatting the SD card, but we still keep losing those 9 daylight hours.

If it's not overheating, what else could it be?

Thanks!