Neptune dryer gas wont stay on

can someone help me figure out what is possibly wrong with my gas dryer?? The gas wont stay on, its on for 30 seconds then shuts off, ity will then fireup again after a minuite or so. I cleaned the exhaust vent to eliminate a possible obstruction. I can remove the exhaust from behind the dryer and the gas stays on. I obstruct the vent with my hand and the gas shuts off. Is it possible the minimul 2 90 deg elbos in my dryer vent is enough to activate the high limit thermostat? Is it just the gas coil solenoids? I just tried to bypass the high limit thermostat and the dryer stayed on. Dose that mean its bad or is it a temp adjust switch of some sort?

Thanks

Rob