The boiler is priming or foaming.
Priming is the violent bouncing of the water. Foaming happens when bubbles form on the surface of the water. Foaming follows priming, and while priming is bad, foaming is worse.
Foaming throws water up into the system and creates water hammer, uneven heating and water level problems within the boiler. If you take a sample of the boiler water and boil it on a stove you'll be able to see if it's foaming. Use a narrow pot when you do this test.
If you see foaming, check the pH of the water. Ideally, the pH should be between seven and nine. High pH (high alkaline) causes foaming and is one of the most common causes of boiler problems.
If the boiler is just surging, it's probably because the water is dirty. Clean the system according to the boiler manufacturer's recommendations.
There's oil in the boiler.
It's very hard to see oil if you're looking in the gauge glass, but it will create a film that causes priming and, in some cases, foaming.
Boiler manufacturers use oil when they thread the boiler's tappings. You use oil when you thread your near-boiler piping. You have to get rid of the oil if you want the new boiler to work well.
Read the manufacturer's instructions on boiler cleaning and follow them until the water stops surging.
There's dirt in the boiler.
When the water is clean, steam bubbles have no problem rising to the surface and breaking free. But when the water is dirty, steam finds it more difficult to escape the water.
As the steam bubbles rise to the surface, they increase in volume. These larger bubbles cause the water line to bounce as they try to break the surface of the dirty water.
Read the manufacturer's instructions on boiler cleaning and follow them until the problem goes away.
The flame is impinging on the metal.
This is a common problem with oil burners and power gas burners. The flame has to be the right size and shape for the chamber. If it's not, it will overheat a part of the boiler and cause steam to form more quickly at that point. As the bubbles of steam race up one side of the boiler, the water on the other side of the boiler will drop to compensate. You'll see this surging action in the gauge glass.
Check the firing rate and flame pattern and correct it if necessary. You may have to add chamber material to solve this problem.
The boiler has very narrow sections.
This is a problem you'll sometimes run into with residential steam boilers. If the sections are too narrow, the rising steam bubbles will lift the boiler water level to a point higher than the level in the gauge glass. This happens because there isn't any steam in the water that's in the gauge glass. The two columns of water (in the boiler and in the gauge glass) sit at different levels until the burner shuts off on pressuretrol's cut-out setting.
When the steam bubbles condense, the water in the boiler falls to a point lower than the water in the gauge glass. The water in the gauge glass offsets this by falling into the boiler. This usually turns on the low-water cutoff or the automatic water feeder. You wind up with nagging water level problems in the boiler.
This type of problem is built into the boiler design, and it's tremendously aggravated by dirt. You can sometimes cure it by underfiring the boiler. That produces fewer steam bubbles so each has more room. But don't underfire to a point where you'll only be simmering the water.
And make sure the boiler is as clean as possible.
There are chemicals in the water.
What sort of chemicals are they and how much is in there? Too much can cause the water's pH to rise, and that will make the water foam.
Check the pH, and lower it if necessary.
The steam is leaving the boiler too quickly.
This is something you won't see, but it's very important nevertheless. If you undersize your pipes as they leave the boiler, the steam will move faster than it should. The high-velocity steam will pull water with it and that leads not only to surging, but also to water hammer, uneven heating, higher-than-normal fuel bills, and water level problems within the boiler.
Follow the boiler manufacturer's piping directions if you want to stay out of trouble.
The low-water cutoff needs a surge column.
Do you know about surge columns? It's a Dead Man's piping trick. A surge column can lessen the surging in the gauge glass and the low-water cutoff.
A surge column looks just like a gauge glass, except it's made of pipe, not glass. You build it from two tees, a few nipples, and a short length of steel pipe, which you'll place between the bulls of the two tees. Hook up your low-water cutoff to the runs of the two tees.
The surge column takes up most of the surging, leaving you with a more-stable water line in the low-water cutoff and the gauge glass. A surge column doesn't solve the surging problem, but it can keep the low-water cutoff from bouncing up and down (and on and off) so much.