If the level is changing with the excessive gurgling in a single pipe durso drain, then you are creating a syphon with the elbow. There should be a small hole in the top of the capped standpipe which, in your case, needs to be bigger. You'll have to drill it out. One method is to drill an oversized hole to allow an airline to fit thru, then put a valve on the topside, for minute adjustments. But the basics is the more flow you have thru the pipe, the larger the hole needs to be to allow air to break the syphon.