Sounds like a bad or weak bake ignitor or tempature probe.....since you stated your broiler works fine i'm assuming you don't have a gas leak because you would of blown up your kitchen by now....with gas leaks it's better to call a professional appliance repairman or your local gas company that supplies your house to have them detect a leak.......If your ignitor is ashy or white in color or cracked it needs replacement it's a common thing for gas ovens....what happens is they heat up for awhile in the beginning and later on the ignitor gets weak and doesn't open the safety valve to let gas in.......video's to help diagnose.
testing oven ignitor - YouTube
http://www.youtube.com/results?......0.0...1ac..11.youtube.