Jagermeister,
You are right for external overflow boxes. If the flow is too low, tiny airbubbles can build up in the U-tube. Multiple tiny bubbles become one big one and the siphon breaks.....
Internal overflow boxes -like the one Christian is selling- don't have this problem.
If you feel the flow is too low, you can also replace the U tube you have with a new smaller diameter U tube to keep a higher flow thru the U tube.