A commonly reported problem when first installing Durso Standpipes is what I call a flushing effect. The water level in the chamber bounces up and down at a slow steady cyclic pattern. This issues typically is not caused by the size of the air-hole in the end cap. The flushing effect is almost always caused by back pressure in the drain line. Typically the back pressure has two main causes:
Drain pipes in the sump being submerged below the water surface to far. Ideally you just want the drain pipes submerged about an inch or two. Just enough to reduce the splashing noise in the sump.
Any loops or dips in the drain lines if using flexible tubing. If using hard PVC pipe for the drain lines make sure they always have a slope to them, don't make them perfectly horizontal or perfectly vertical.
The flushing effect works like this: Back pressure in the standpipe prevents air in the pipe from exiting the drain line. Instead of exiting the drain line, the air bubbles try to rise in the pipe slowing the rate at which the pipes drain. The water level in the chamber then rises. The increase in the chamber water level adds pressure to help clear the air from the pipe. Once enough water pressure exists in the chamber to overcome the back pressure, the air is literally is "burped" out of the drain line which crates a sudden rush of water. This is displayed as a rapid drop in the overflow chamber water level.
This cycle then repeats itself over and over as the back pressure builds and gets purged.