I'm not sure but it sounds like emotional stress is coming a need to impress your wife's friends. You're comfortable when in your domain at work - even if it's strangers on the phone, etc. but the pressure to be whatever kind of person you think your wife and her friends want might be tugging at your self confidence, making you uncomfortable/nervous to perform and therefore stammer more. It's best to figure this out with a speech therapist or a regular mental health therapist. It seems like your speech is fine otherwise, it's just some stress causing blocking.