You might supply the make and model and size of the RPZ device that's acting up. Some RPZ's might act flukey before you actually are running them at normal operating flows. The likeliest suspect here is a blockage of the water passage to the relief valve diaphragm. When that's blocked, the relief valve will open wide, and a gusher will result. By the way, it wouldn't hurt to bleed out the air from every one of the downstream test valves.