Oven burner not lighting normally - strange behavior
Could be a cracked ignitor element, letting through enough current to open the gas valve until it gets even hotter, then the crack widens and the gas valve closes. 

Could be a faulty valve, and the valve throat is blocked by debris or broken part (as opposed to valve coil is broken). 

You can distinguish between those cases by reading the voltage across the valve terminals while the oven tries to start. A steady rise in voltage with no spikes or dropouts means the ignitor is OK. 

Sometimes you can detect a cracked ignitor element by direct inspection -- it will have an obvious hot spot, which often looks like the element is burning. 

If you can't see the ignitor or measure the voltage, you could try replacing the ignitor. It's the cheapest part, and any replacement is cheaper than calling a licensed repairman.