Internal Fan runs at hi speed

Recently the internal fan started running at hi speed after start-up of my iMac 27". Runs that way for a period of time (30 mins?) and then shuts off. Doesn't run again until next start-up. Had this problem before and took the computer to the apple store. They said they couldn't find anything wrong but when I brought it home the problem had disappeared. Now it is back. Have tried resetting the SMC numerous times with no results. Does anyone have a solution other than resetting the SMC? This is frustrating - I really like my iMac except for this intermittent problem.