Arduino yun fails to boot

I have 5 arduino yuns running as temperature/humidity data loggers, using the bridge to communicate with a python script that generates rrd graphs out of crontab.

Occasionally, the yun will crash with the green Power indicator on solid, and the white USB on at 1/4 brightness. There is also a high-frequency hum audible when this happens that is not noticeable when the yun is running normally.

If I unplug/replug power at this point, 9 times out of 10 it will immediately power up with the same lights visible, ie. green at full power and white at 1/4 power. Eventually, it will power up with the green light at full power, the white light will flash very briefly, and then it powers up normally and runs fine until the next crash.

I’ve tried a number of different power supplies and also POE modules, all of which failed eventually.
I’ve updated to the latest openwrt image, and it hasn’t helped.

Curiously, I have one yun which has never failed in this way, it has been running for months without errors. It’s running a very similar sketch, but not an identical one. Same python/rrd graphs as all the others.

Any ideas what could be wrong here?

Thanks
Rick

Well, it seems the yun without problems has a slightly different sketch. I would think that would be a good starting point?

Do all the yun's have the same image, including the u-boot stuff?

Does it continue to crash if you disconnect all the hardware from the Yun (sensors, etc...) ?
Is the hum coming from the POE module or from the Power supply?