I have a Thermador PRSE304/48 with 6 gas burners. The burners all spark but won't ignite. I do not hear or smell any gas coming through. After a minute or so of trying, all burners suddenly start working and I can turn them on and off just fine (though gas flow seems low at first). If I then turn the burners off for a minute or so, the same issue of no gas flow happens. I can understand that if one of the burners won't ignite it is probably a gas control valve problem, but all 6 at once intermittently not working has me stumped. Any ideas?