[heavy chevy]

Have lpg gas range,both large cooktop burners have low flame .I"ve owned this range since new.I have tried several things to try and fix. :cleened buurner ports ,burner tops,orvices,ect.these are the only burners used ,it takes forever to boil water,small burners work fine,gas pressure is ok,wondering if gas controls are getting worn out or buners getting old ,I've checked truness of burner tops seem alright burners are slightly warped .this is a seal top burner set up ,any suggestions are welcomed THANKS