sounds like your return pump cant keep the water level high enough to keep the overflow box syphoning. you might want to try a bigger pump. depending on your overflow rate (how many
GPH you box can handle), you should try to match that as close as possible to the return pump you are using.
example ... if your overflow is rated at 600 - 700
gph the a mag 7 return would keep the water level about right. of course you need to subtract head loss and elbow loss, but a mag 7 would work in the example.