how to restart OSX every 3 hours automatically

couldn't you just plug the computer into an electronic timer which switches off and on again every 3 hours (the kind you use for lights to keep burglars away) then you can set the preference for computer to automatically restart every tie the power cuts out.

Yes, you could do this if you want to create cumulative errors in your hard disk which will eventually render the whole system inoperative. Every time you force quit you are extremely likely to cause disk errors, and if you have to do this as an emergency measure you should run a Disk Repair. You should not consider doing this on a regular basis as suggested.

I agree, shutting of the power is risky for the hard drive. If this is a laptop with battery, then can't it just safely shut down after a specified time on battery via energy saver settings, or it will do that as the battery runs down?
And maybe for a desktop, what if the computer was plugged into a UPS (with computer interface via USB), with drivers/software to safely shut down the computer when on battery backup power after a specified time? When the wall power goes out the UPS can shut down the computer gracefully.

I haven't used that feature myself with a UPS, so that's just speculation on my part here, but I think that the APC UPS's come with a USB interface and should work with Mac OS X.

Glen - he wants to reboot automatically, not just shut down.

dgd - have you tried my script to see if it works? possibly with the 'tell application "Skype" to quit addition?

If indeed 'tell application "Skype" to quit' won't work while the dialog box is there (something I can't check as I can't see a way of deliberately crashing it) then it may be that QuicKeys is your only answer: this can detect the presence of the dialog box, get rid of it, and initiate a reboot, so that in fact you would only need to reboot when there had actually been a crash.

i know everyone would love to get the thing behaving as intended, and i did try to suggest that my idea was a last resort approach, but he's using some old doorstop of a powerbook that no doubt caught a lucky break on it's way to the parts bin to become the worlds most over-specced baby monitor, and i'm sure he'll be happy enough to wipe and re-set in 12 months time if / when it packs in from multiple power outs. if it were me, and i'd got it doing what i need i'd spend my time fixing a multitude of other bugs on my macs that need to work properly all the time, and leave that powerbook to a hard life for the next year or two until the guys kid is old enough to restart it for him, my 3 year old nephew is pretty handy on the mac already.