If the broiler works fine, but the oven does not, more than likely this is caused by an igniter that has gone bad. The intermittent symptoms you were experiencing is a sign that the ingiter probably became weak due to age and finally failed. The following link explains how these igniters work and how to replace one:
Since both oven manifolds are fed from the same gas source, this can be narrowed down to an igniter problem. Your oven should have the Norton type igniter. The part number for a replacement is 9782065 and can be found at appliancepartspros.com for about $71, You can search around if you like, but this was the cheapest I could find it.
Your oven has interchangeable igniters. Meaning the broil manifold and bake manifold use the same type of igniter. You can swap igniters while awaiting parts, if you wish to use the oven to bake.
NOTE: If the connector plug on the ingiter you receive does not match the receptacle in your stove, you will instructed to cut the leads and connect them together using ceramic wire lugs (included in the kit you receive). Do not attempt to use standard wire lugs as they will melt. The wires and ceramic lugs are heat resistant.
If you have any questions, or need additional assistance, please let me know. I hope this helps you.