crontab -e
Add the following line to the file:
* 0,6,12,18 * * * /sbin/coldreboot
Hit Ctrl+X to exit
Y to save.
With that said, I agree that if it needs rebooted so much then you are overclocking too much or perhaps one of your cards is defective.
I have two rigs, one with 4*5850 and one with a single 5850 (and a ztex fpga) running bamt and both seemed to lockup even without major overclocking. Could be a faulty card but both rigs where running for weeks with ubuntu with no lockups. "why did you switch from ubuntu?" Mainly for the mgpumon and but having learnt more about 'screen' etc I'm thinking of switching back.