You might try adjusting the depth of the air tube up and down in the drain pipe. Or try increasing the size of the air tube.
You're getting a siphon in your drain and introducing enough air at the right place will break it and not allow it to form.
Also, since you're using only one drain, make sure it has the capacity to drain what your return pump is pumping.
And, imo, constricting your drain with a valve is a bad idea.