there are units available to test delay boxes but they're probably a couple hundred bucks anyhow. i think most manufactures will test them cheap if not free but i've never had to send one back. if you're really suspecting the box i'd get a known good box off someone and try it. worst thing is seems like you're dealing with an intermittent problem which is your worst nightmare, hard to trouble shoot something if it's not doing it all the time.