kenmore dryer shuts off

I am having a problem with my Kenmore dryer, model 110.63102101 shutting off after about 20 minutes of drying time. It started about 3 weeks ago. I took the back off, cleaned it all out, cleaned the lint duct, whatever I could reach. It ran fine for a couple of weeks, but now the problem is back. From a cold start, it will run normal for about 20 minutes, then shut down. Restarting it after about a half hour, it will run for about 5 minutes. If I immediately restart it, it may run for less than a minute, or not start at all. If I come back after a while, it will restart, but run only for a short time. Something inside needs to cool down first.
This condition is in high heat, and low heat is not much better. I can run a normal cycle in air dry.
Any suggestions?

I had a Ge Spectra with a broken igniter glow bar. Turn on the oven and look in after 10 - 15 seconds. If you don’t see a glow coming from the bottom of the oven, the igniter is probably broken. See the following posting for instructions on checking and replacing an igniter.

http://paulgrif.livejournal.com/1754.html