The GC should be able to operate at 35C per minute all the way to 350C.
The following items would cause this problem.
The power voltage is less then 110V. uncommon issue
The Triac in the GC is faulty and needs to be replaced. Most likely open a service call or replace the AC board.
The oven heater is not working correctly. less likely
Be sure to unplug the GC from the wall outlet before opening any side covers on the GC .
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One simple thing to check might be to start the oven ramp (or method, in other words), and then walk around to the back of the GC and look at the flapper. This is the exhaust door that opens and closes to let hot air out, so if it's not closing during the time when the temperature is increasing, it won't be able to maintain the ramp and you'll see the behavior described. This should be easy to observe.
I agree with Valentin, it is much more likely to be the flapper.
I am marking this as assumed answered, if you still need more help, feel free to comeback and post.