Do you have the overflow hooked up to a venturi powerhead or an aqua-lifter pump? Have you looked into getting one of the those aqua-lifter pumps? Here is a link to the MOPS site where they sell them.
http://www.mops.ca/cgi-bin/SoftCart....b7d+1116695876
When I was running the same overflow, I used to keep an eye on the water level, and when the water started to get higher, I just sucked any air out of the overflow that was trapped.