Just got my 24 year old Force 10 refurbished. Works like new now. None of my burner valves were frozen but one (the big one of course) had the same symptoms as your third burner. It was as if the thermal couple was not working although sometimes it would stay lit. Spoke to the service
guy (anybody in the Toronto area need theirs fixed, the guy is great) and he said the thermal switch was sticking. He explained it all about a little ball fitting somewhere and it was dirty. Highly technical but his main complaint was that it was hard to get at. But by the sounds of it, could be done by a layman.
I was on the verge of getting a new stove and am really glad a friend referred me to the gas mechanic
. Two hours labour and it was like new.
What age is yours? Does it have the brass burners that lift
off? You can do a lot yourself. A bit of lube on the sticking valves will fix that. Remove all the orifices (I usually call them jets because they look like carb jets) and soak in alcohol to clean. Do not use wire as it will damage the orifice. A 7mm socket does the trick.
Force 10 might have parts
if you need some. They are quite helpful.
FORCE 10 - Cooking Without Compromise