Does it happen when you have the burner on full noise, and do you notice the sides of the bottle getting really cold when it happens?
Maybe it's the flow capabilities of the gas bottle not being able to keep up as the boil progresses?
With higher gas flow rates, sometimes bottles have a little trouble keeping up, in some cases you get icing/freezing occurring on the outside of the bottle. Are you using a 9kg bottle, or a smaller one?
Try sitting the bottle in a bucket of water so it has a 'heat sink' to absorb some of the chill the bottle is trying to lose to keep up the gas flow.
I particularly noticed this when I was using my gas forge with a 9kg, the bottle would ice up and the flow would drop down to the point the burner would actually go out. I got a 45kg bottle instead, and never had a problem again.
Not sure if this is what's happening for you, but possibly. There's an equation that works out whether a bottle should be able to keep up with flow demands from burners, but i can't remember it. Maybe there's a gassy here that can explain it. Something to do with the diameter of the bottle I think, that allows the LPG to tun from liquid back into gas at the required rate.