Yep there you go. Your return pump is not strong enough. Your return pump decides the flow of the overflow box. If your overflow is rated at 800gph, then your rio is not getting anywhere near it. I didnt look it up but i dont think that pump goes more than 300-400gph. You need to at leat match the 800 or have more so that you can have room for adjustment.
When a pump flows water upward, it loses power and the flow rate decreases. With ever foot of overhead (head) that your pump climbs, the gph
output is decreases. So if your pump is rates at 375gph and it travels from your sump, 4 feet above the rim of your tank, then your final output might only be 230gph.
The final output for your 800gph overflow needs to be over 800gph. I would recommend a mag9 which is 900gph but with the four foot head should still give you around 800gph.
When there is not enough power in the overflow airbubbles are created and once it builds up the siphon will fail and stop.