How Can I Fix My Hygena gas oven AHY0142 model 050502079?

Ive got a Hygena Gas oven AHY0142 Model 050502079. when oven in use seem to hear fan working as if labouring (going slow) or sometimes not at all,but once been on a while seems to sometimes come back to speed.also noticed that sometimes when 1st turned on gas flame going out on its own and then you'l hear igniter sparking until it re-lights again?? Also if I use the grill, when 1st turned on flames on grill seem to come towards front of oven (nearly out the door!)once been on a while seems to then work normally??

Any ideas whats wrong with it?? as would consider having it repaired by my local domestic appliance repair firm if its worth while - would like to know whats wrong with it so I don't end up getting 'ripped off'

Thanks