Why does a fan keep turning on and off every few seconds

Hi!
Usually what you'll find is that the fan will turn on and off as the computer heats up and cools down. The fact that it turns on and off every few seconds could imply that either: a) Your computer heats up and cools down very often (not likely) b) Something is causing your computer to not sense the correct heat c) There is something impeding your fan from working properly (most likely).
I would recommend using an air duster to blow all of the dust out of the fan area and see if that improves anything.
Additionally, you can download free software which will monitor your CPU temperature, and that should let you know if you're within a reasonable range (Google for specific numbers).
Good luck! -SJ

Laptops and desktops all the same need to have cleaning maintenance every 1 to 2 years since their components will be accumulating dust with the pass of the time. Your laptop fan will keep turning on and off because of the constant heat of the CPU since the fan have accumulated dust it will not work as expected having to be force to work with more load to keep your CPU cooler, that fan doesn't stop completely it only lows the rpm (revolutions per minute) but since it can't keep it for much time then is when you hear the excessive noise of the fan working.

open your laptop or have some with knowledge to do it for you to have the fan cleanup, that will extend the Laptops life.