We have a problem keeping the LPG gas burners alive after ignition?

We have a SMEG SUK91 cooker, with 5 LPG gas rings and an electric oven. After we've ignited a ring(s) they sometimes go out and will not stay alight after reigniting. At other times they ignite and stay alight working fine. The electronic sparking control knobs appear to work fine. The flames do seem low, less powerful, compared to how they used to be. We have plenty of gas and do not have any problem with the gas boiler or fire. The oven turbo fan doesn't work either. Is there anything I can do to try and cure the problems, especialy with the gas hob? Thanks