IMac boot hangs/takes a while

So I've had my iMac for almost a year (and opted for Apple care which I'm glad about!) and within the past month approximately 3 times the boot sequence has hung. It either hangs at the gray screen (with or without the apple logo) or it gets past that and too just a purple screen right before my desktop wall paper would normally appear. I let it sit for a minute or two and it didn't finish. To get it to boot when this occurs, I either turn it off and back on or sometimes I've resulted in booting from the DVD and running disk utility. Disk Repair finds no issues most times but repairing permissions seems to fix it when it occurs.
It just started doing this and is otherwise working fine. I've had the extra 2gb of RAM in for quite a while before this popped up so I don't think it's related.
Any ideas? I'm running 10.6 but it first did this with 10.5.7 I believe.
Thanks
Dave
Message was edited by: dfbennett

Thanks for the tips.
Since it's an intermittent problem I'll have to wait until it springs up again. I did a bit of research and found that some folks have had this problem with a USB hard drive plugged in while booting. I have a regular old IDE hard drive in a USB 2.0 case plugged in (it has switched power) and sometimes it's on sometimes off when I boot. I'm trying to see if I can find a pattern to the issue when the hard drive is off versus on.

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