On Fri, 09 Jun 2006 18:52:56 +0000, Scott Sayer wrote:

> Exact same thing happened on my 67 Fastback. After much consternation, it
> turned out to be the ignition module inside my new Mallory distributor. When
> I checked it in my driveway using the Mallory diagnostic check, it checked
> fine, when it heated up after driving, it didn't. Change the module (not
> cheap) and it should be fine.
>
> -Scott

There ya go!
Sounds like the OP's might still be under warranty.
Doesn't hurt to give them a call and beg for mercy!