Burnt contact inside dryer most likely
Believe it or not if you are very careful you can fix the contacts inside some of these type dryers. What happens is the timer has a terminal which connects directly to the heating element. It is a simple design. Most repairmen will replace the timer and rightly so. But I was raised by depression era parents that learned to repair stuff not replace it. If you wish to save a few bucks you may want to try and clean the contacts inside the timer. Another thing to look at is the appliance receptacle and 30 amp breaker. On rare ocasions that breaker will fail on the heat side. The motor is 120 volts and runs off of neutral and one side of the source. So it can fool you into believing everything is kosher in the electrical department. These are some things to check into. For the heating element itself to be intermittant no way. It is either gonna work or not. No in between.