3 beeps pause 3 beeps pause 3 beeps pause 3 beeps followed by shutdown. Intermittent. Not reproducible.

I've a mid 2010 imac with intel i5 cpu. Once in a while, I get the following sequence "3 beeps, pause, 3 beeps, pause, 3 beeps, pause. 3 beeps. Shutdown". It's not reproducible. Whenever it happens, the sequence is the same. Have taken it to the Genius Bar. They ran all the RAM and other tests, and couldn't reproduce. Have tried to reinstall the OS as well, still no change. Please help.