I was dissapointed to find my normally very reliable Lady Anne surge and accelerate, without RC input, so badly at RailExpo that it very nearly left the baseboard and landed on the floor. I subsequently ran it on rollers and observed the surging process repeating without any glitching of the regulator.
I have subsequently received this reply from a fellow modeller and can confirm that I ran the loco without removing enough of the 300ml of airspace, as recommended, and ran with a lot of excess pressure emitting from the safety valve for effect. Anyway this is a new one on me and I include the reply as it may be of interest.
In my experience 'surging' as you describe it could generally be tied to two possible causes. First, the water in your boiler at 40 psi has a temperature well above the standard 100 degrees C. of water boiling outside a boiler. Two possible causes of the phenomenon,
then: in the first the boiling water sends up a slug of water from the surface of a too full boiler which (inconveniently) gets sucked into the steam line. Once past the regulator and outside the high pressure environment of the boiler this hyper-heated water flashes to steam. This uncontrolled burst of steam hits the cylinders and causes a surge of acceleration. In the second, when the safety goes off it causes vigorous action at the water surface often sending up slugs of water which can follow the same path, with the same result, as described above.
There are several ways to deal with this problem The first is to put less water in the boiler. This is where the top-up valve comes into its own. Another is to run at less than the 40psi at which the safety goes off. Both will contribute to a relief from the surging.