My overflow drain has gas

[Richer]

Have you tried increasing the amount of air that gets into your standpipe? (ie. increase the size of the air hole). I found that when I was pushing lots of flow through a standpipe I'd also get the flushing effect you see in youre pictures... after I increased the amount of air that went through the pipe, the flushing stopped altogether, and the level in my overflow rarely ever moves up or down.

*edit*
Looks like Tony get there before I did

-Rich