IPhoto problem with RAW

[Khandakulu]

My iMac generally locks up if I try to do more than one crop exercise on a photo. The only safe way to crop is to do one crop movement at a time, and saving before the next. My iMac has the 2GH Intel core 2 duo processor with 4GB SDRAM. I have a Panasonic Camera (FZ200) and save most images in RAW. (12 MP files - not particlularly large RAW files) . When the iMac locks up I can only hold the start button down until the machine switches off and reboot .