What size bulkhead fittings do you have and what size pipe did you use to make the stockman overflow out of?
I just went thru this myself for the first time, I have a 1" bulkhead fitting and used 1 1/4" pipe to make a durso overflow. On top of the durso I drilled a 1/8" hole to even out the water level. It was consistantly going up and down verses staying at one level in the overflow. After I drilled the hole the water level stayed constant and there is no slurping noise or any noise for that fact.
Possibly your overflow pipe is to big for your bulkhead? Just taking a stab at it here....
Both durso and stockman have web sites with tons of information on them. I thought durso had a easier set up than stockman so that is how I went with mine....