Gas Dryer doesn't stay hot

Heating element glows, gas is let in and flame lights. However it stays on for only approx 25 seconds and then off for 2 minutes and then on again for 20 seconds and so on. Something seems to be telling it to turn off and we can’t seem to find the part necessary to fix problem. Took it totally apart and cleaned/ vacuumed out everything. Need advise please.

Roper dryer doesn’t stay hot