You issuse is that the return area of your sump is much too small.
In other words, the amount of water needed to be pumped to the display tank for the overflow in the tank to function properly is greater than the amount of water the return area can hold. This causes the return area to empty, and your pump run dry.
As an example, take a look at this manufacturer's sump (offsite link) -
http://www.superskimmer.com/48_Sump.htm
Note that the return area on the right, is about 1/2 the entire sump. It's true that this is a sump specifically designes for a reef system and in that application, that return area may have other things in it and the flow through it tends to be much greater that what is typically used for a planted tank, but you get the idea.
I think you best long term solution would be to rebuild your sump, making the return area larger. You might want to use either a larger tank or two tanks for the sump to get in every feature you want.
You could restrict the flow from the return pump, but that's going to be counter productive, and the water level in your main tank will be a bit lower.