did you check the thermostats for continutiy should read 0 ohms with digital meter. You check them with out power and uninstalled. It is probably the lower temperature one because it cycles the element off and on. Good Luck

it sounds like another gas valve coils went bad. Usually when they get bad they work for the first 15-30mins after you start the dryer then quit. then works again after restarting you dryer. then quits again and so on....

Open the access panel on your dryer, back or front of the dryer depends on what model. Let the dryer run and observe if the igniter glows, observe if gas valve opens resuting burner to lit. Observe for several cycles, if you noticed igniter to glow but no resulting flame from burner i would suspect solenoid coils are bad, easy to replace.

edit:
opss...you said heating element. must be electric dryer. my bad.
check thermostats for continuity.

Have you checked/cleaned all the duct work in the dryer. Also did you clean the vent from the rear of the dryer all the way outside?