Cannon cooker fault?

This is the fault I have with my Cannon C60ETX. I switch the oven on and the fan starts to pick up speed but 'pulsates'. The fan speed keeps dropping, then picks up again quickly and you can here an audible thud as it does. If I put my hand on the outside of the main oven door I can also feel it as it pulses. It's as if the fan is losing power. It carries on like this until the oven is up to temperature then appears to settle down but still occasionally does it.
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Using the top oven initiates the cooling fan (as does the main oven). I listened carefully and the cooling fan was also pulsing and constantly losing speed as the top oven was heating up. I also noticed that the cooling fan speed only pulsed or lost speed when the grill element came on. As soon as the grill element was switched off by the top oven thermostat the cooling fan behaved normally? This is the same for the main oven. If I turn down the temperature the fan speed is normal, ramp up the temp and the fan is erratic.

I'm struggling with this one. Any help would be appreciated.