stoves gas oven switches off before reaching the required temperature?

I have a Stoves gas double oven with the following identification markings:
STOVES Q720 GRF D0
Mod: 059050517
S/No: 000634
Natural Gas 5.5kW
The oven comes on but switches off before the required temperature is reached. The indicator light changes from green to orange. After a while the oven comes on again the process repeats itself. The fan is running corretly. The grill/oven oven works as it should.
i would appreciate any help.
thanks