Overflow Question

[intarsiabox]

I have just set up my first tank that has built in overflows. It is a 52g tank with overflows in each corner and a 1" Durso stand pipe in each. It empties into a 20g sump. Every now and then only one of the overflows seems to fill up more than usual and then empties out really fast causing a flushing sound. It will do this once or twice and then stop for a few hours. I have a Mag9.5 return pump going into dual 1/2" loc-line returns. I had the pump running at full discharge for a bit but turned it down by 1/3 until all micro bubbles stopped coming into the tank. My problem seemed to happen more often when the return was at full discharge.

Do I need a bigger air hole in the top of my Durso? Should I dial the return pump down more? Any ideas would be great!

Thank-you!