dryer takes 2 to 3 cycles to dry clothes

My dryer is taking 2 to 3 cylces to get the clothes dry, I have clean out the venting, put in a new heating element, also changed the high limit thermostat, and the operation thermostat, and some other thermostates, It is like it heats up some but then goes cold, could it be the timer?

thanks
Calvin Peck