Halve the frequency (75 MHz) and see if they run longer than 6 hours.
No, still having the same problem at about the same frequency (no increase at all in how long they run).
Also (in less than a week) cgminer has crashed four time (I just see "segmentation fault" with the terminal back at the shell prompt.
This is starting to sound similar to what btckess experienced.
Follow his troubleshooting step(s) up to resolution from here to see if any of it relates:
https://bitcointalk.org/index.php?topic=1764803.msg18074121#msg18074121I'll take a peek a Gentoo this weekend.
I use to enjoy using that, but have nearly forgotten it still existed these days.