Fault on BG437 White Knight Gas Tumble Dryer?

When the appliance is switched on it will sometimes ignite and works fine but on occassions the flame goes out after a short time and it then doesn't re-ignite and therefore just runs cold it seems very hit and miss. In the past I have had problems with the solenoinds and I have replaced these but this hasn't solved the problem. Has anybody any suggestions as to what the problem could be. Thanks.