It is a bit hard to tell from your post, but does sound like problem is
with thermostat. This is not expensive to replace, if necessary.
Needless to say, be very careful if you are really talking about about
problems with gas burner, and you're not experienced fooling with
these. Might be good idea to spend a few hundred on a plumber/ heating
contractor, at least call a _knowledgeable_ friend.