Couple thoughts:
1) Your street El on the durso is pretty high up which accounts for the high water level overall.
2) If this is the used setup and you redid the under stand plumbing then I would bet that the length of the drain pipe and how much is under water in the sump has changed from the previous owner. These would cause a higher back pressure in the drain and cause the 'flushing' effect. Your probably going to have to increase the air hole size. You can experiment by using air tubing and shove one end in the EL part way down the drain and see if the extra air helps, then add another etc until you get it where you want it. Then either leave it, or take em out and start increasing the hole size until you get what you want. Flex pipe doesn't matter as it's not the unions/corners/etc that you're worried about here.. It's length and back pressure.
Anyway, that's my take from the vid
