I hope I never experience that kind of rollercoaster up & down the Cum Paste guy went through. Just like you, Martian, I was laughing and cringing at the same time. That bit took off when Artie chimed in to advise the guy to get an agent. He was also the first to notice how the guy didn't believe they were goofing on him, as opposed to the fake praise they heaped on that horrible bit.
The other thing is that it completely overshadowed that REALLY awkward Sal comment immediately before they got the Cum Paste guy on the line -
Howard - Calm down, dude.
Sal - Well, you're a tyrant sometimes, so y'know...
Howard - Yeah, I'm real hard on you.
Sal - I wish you were.