Herbie Help

[neoh]

I would also consider a durso mod for the splashing and silence.

[Skimmin]

A couple things to try.
Where is your gate valve located? Depending on the location it can potentially trap air and cause throttling issues
Try a different return pump. You could have a bad pump that is fluctuating how much flow is going into the tank.
Try a different valve. I personally had a pvc gate that I could not keep a constant level with. I switched to a true unoin ball valve and never had another issue.
Does your drain run vertically into the sump or is it horizontally offset? Large or too level of an offset can trap air causing fluctuation in the siphon.
Can you put up a couple more pics of the plumbing as it leaves the bottom of the overflow box and enters the sump. I think your issue is more likely drain related vs pump related