Gas will ignite when dryer first started and dryer will heat up. After a little time, dryer gets cool and will not reignite. After leaving dryer off for a bit, same thing. Heat, then no heat.

I have had the back off. Venting is clear and I have checked the cycling thermostat, thermal fuse and high limit thermostat. I've never done this before. The directions for the tester I bought said when set on RX1, they should read zero, which all three did, except when I put the the cycling thermostat on a heat source, then it read infinity. The only odd thing was when testing the high limit thermostat, it didn't read zero right away. I had to leave the tester probes on it for a few seconds before the meter went all the way to zero (totally a novice here so don't know what's important).

Needless to say, put the dryer back together and nothing solved. I've read a few posts and saw gas coils mentioned. Where should I start next?