No a pre-heat is not required, it sounds like the thermostat is not working properly.
Its worth buying a oven thermometer and checking what the reading shows compared to that of what the oven temp is set to.
Let me know what readings you get and we can go from there.
Hi Peter, I havent heard back do you require any further assistance?
I havent heard back from you please let me know if you have any follow on questions.
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