iMac problem

This appears to be a RAM (memory) error. Have you installed any new RAM recently? If you have, try swapping the module configurations around and repeat the test. It sounds like one of the modules is defective. Once you've found it, get rid of it: whether this means returning it to the manufacturer for a refund, or just binning it - 'cos it is kaput.

If you haven't installed any new RAM, take it to an Apple Store - they'll know what to do, but it ain't your fault.