Years ago I had a similar problem with a gas fire. Flame would slowly decrease to almost nothing and then suddenly flare up again - perhaps five second cycle.
It turned out to be what the gas man called, "water in the service" - We listened to the supply pipe to the meter and could hear gurgling as the gas pressure forced the water up the pipe and then the column of water collapsing when it was too heavy to be supported.
They pumped the water out. No cost to me. Listen to your meter when the cooker is on.
Peccavi
October 2008