Every time our washing machine hits the drain cycle, our kitchen sink gurgles and begins to smell of sewer gas terribly. We were told it was probably due to a clogged vent. My husband went into the attic and checked for clogs. He did find a cap on the pipe, which he removed - and the sink didn't gurgle for a couple of loads - but now it's back at it... just as bad as before. He's checked the pipe again, and can see clear down to the bend in the pipe. He also replaced the Stutor vent under the sink. Any ideas on what this could be?

Cap on the pipe?? You got a mess going on here, vents are never capped so who know who's been working on your home. That vent should also terminate above your roof line. For now, get a sewer rodder and rod the vent stack to the basement plus 20' . You smell sewer gas because your clogged vent is siphoning the water out of your trap. Add water to the sink immediately after the washer drains to hold you over until the repair is made.

Bob nailed it!
What's happened here is at one time you had a clogged kitchen vent the plumber couldn't clear so he capped it off and installed a Air Admittance Vant,(AAV) in its place. You have two options here.
1) Cap off the open vent in the attic and install a working AAV in the cabinet **OR**
2) Clear the vent and reconnect to a existing roof vent using a inverted sanitary tee.
Good luck, Tom