Yeah - You have the same tank I had (have.) I ran a herbie for several years, then when I had to move the tank I cut out the corner flow, drilled the back and put in a coast to coast with a bean animal system. Basically, your options are having an open standpipe, a Durso or running a Herbie with an external return. From your description, I think the pump is overwhelming the capacity of the Durso and causing it to intermittently siphon, making the level oscillate. If you’re happy with the flow you have now you can just leave it. If you want more flow, I’d covert it to a Herbie.