Have you checked the exhaust run? Some dryers will start to heat and if the exhaust line is plugged it will trip a thermal sensor. After it cools a bit it may start to heat again. This cycle will repeat til the timer goes off. Obviously this increases your drying time.
Clean the lint screen and make sure the vent run is clear. If you use that white plastic junk get rid of it and replace it with 4 inch galv. duct. If it is to complicated to DIY call a CSIA chimney sweep(i know, its a dryer but it is a vent) many of us also clean and repair dryer vents too. They are a little known source of home fires! Good luck.