Shut the pump off and let everything equalize...Make sure your sump is full.
(simulate a power off situation) restart the pump, and see if that does it...
As mentioned above, you might have a pump that has too much gph for your overflow(s)...there really insn't a way to increase the amount of water overflowing the tank, other than adding another one.
You can, however, use a valve to slow the pump down from returning so much water to the tank...
What is best, is to list the gear you have and post some pics, so that we can see what we are working with...