> Sorry, but a negative reinforcement cycle of tensing up in a medium stress situation is exactly the kind of characteristic that I'm trying to screen out in an interview.

Then you are doing it wrong. There are different types of stress. Holding up well under "stress" is a misnomer, as someone can hold up well under one type of stress, and completely fall apart in another situation entirely.

What you are testing for is the ability to handle the stress of interviewing. If anything, you are hiring for people who can more easily leave.

While it's appropriate to test for situations where stress is a factor, it's equally important to realize that talking to a customer and doing pre-sales and an interview are about as far apart as possible on the stress meter. One hardly equates with the other.