[untamed]

The bulkhead and pipe can handle the volume...but...it's about head pressure and flow rate. More head pressure = higher flow rate.

As the water rises, it builds head pressure against the overflow until the flow rate increases beyond the rate of the incoming water. As soon as that happens, the high flow draws down the water level, the pipe gulps air...disturbing the flow rate which then allows the water level to start to rise again.

A 2nd overflow hole would solve your problem, but I'm sure you aren't wanting to do that....so my suggestion is to run a siphon pipe from there into the sump below. Put the intake of the siphon lower than the current outflow. I'm sure a siphon will make everyone cringe...but you always have that other overflow hole to run on if the siphon were to fail.

No harm in trying it, anyway. You can test if it works with a big piece of flex tubing or something.

Believe it or not, another solution would be to put a gate valve on that pipe and close off the pipe until you get a steady state at which inflow matches outflow. Unfortunately, this is a formula for a flood unless you have that 2nd overflow hole to absorb any change in flow.

B