Are you still running one at a time? Are you able to run one alone at 200 MHz at 3 o´clock? I think that 3 Amps isn´t enough for 250 MHz to run stable.
What happens after 24 hours? Do they go zombie?

In my experience putting them into a box isn´t the best solution. I tried this, even with an open case and two fans. They get hotter than with my current setup, open just inside the hub with the arctic breeze fans.