Overflow problems

[eli@fijireefrock.com]

Agree with the above.
The water level fluctuation in your sump will play a big role on the pressure of your return pump giving you different water levels in your overflow box and it seems you let the water drop too low in your sump before replenishing the evaporated water.
By now if your sump is one compartment best is getting a ATO and a gate valve is best.
Or if you have different compartments in your sump and your return pump is in one try never to get the water level in the sump go over the compartment separation level for the return pump as this will void its job of keeping the main tank water level in check.