I'd say that you have a weak ignitor. The amp draw of ignitors weakens over time. Once the current flow drops below 3amps the gas bimetal either will work inconsistantly or not at all. This is a safety feature.

If you have an amp meter attach it to either of the two wires to the ignitor (use the 4A scale).
Turn the oven on.
If your current flow is less than 3.0 replace the ignitor. It should reach it's full draw in under a minute.

If you don't have an amp meter and cannot do the correct diagnosis replace the ignitor first. In my experience, over 90% of people with your complaint need an ignitor. The odds are in your favor with an ignitor. In contrast, valve replacements are very rare (poor odds).

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