When the Richmond Hill Barn Theatre's current production of Arsenic & Old Lace is really rolling, this venerated black comedy (well, dark-gray comedy) is absolutely delightful. The problem, though - or rather, the problem with last week's opening-night performance - is that Richmond Hill's production doesn't so much roll as lurch; despite their frequently endearing characterizations, the actors on Thursday evening had so much trouble getting their words out that the show never quite found a satisfying rhythm. The experience was like taking a vacation road trip in a vehicle that keeps threatening to stall: You've made it to your destination, and you've had a good time along the way, but you're still a little grateful when it's over.