Fan Bursts- help!

Hi, if anyone could help I'd be grateful. I'm having fan issues on my iMac (details below). The fan keeps coming on for very short (1 second bursts). Does this six or seven times every minute. Sometimes to full speed, sometimes a barely noticeable purr. Always in bursts. In layman's terms, I'm basically word processing, web surfing and emailing so I don't know what I'm doing that could tax the system. iMac is well ventilated. Have tried resetting it by unplugging the power cable. But no dice. Any ideas for a fix? It's infuriating.

Thank you to anyone in advance.

OSX 10.10.4

iMac 21.5 inch, GHz Intel Core 5

8GB memory MHz DDR3