Over 90% of the time your problem stems from inadequate air flow. The hot air is building up inside of the dryer causing the thermostat to cycle off and on. I will assume that you clean the lint filter after every load. These filters do not trap 100% of the lint. As time goes on the lint continues to build until it clogs completely. First remove the exhaust duct from the rear of the dryer and make sure that it is clean. Next remove the back panel on your dryer. Inside you will see a duct that leads from your lint filter to the exhaust duct. Try to run a long wire (wire clothes hangar opened up) all of the way through the duct. If this is jammed then you will probably have to remove the duct to clean it thoroughly. If you found everything clear then you can proceed to the other checks.
1. Broken fan belt 2. motor fan and bearings 3. Check the felt or plastic seals around the drum 4. Thermostat (use a VOM)