Thanks all again for the help. I love the arduino and the support of this forum is priceless! I did have the interrupt pin connected to both a button at one point and later another IC that has an interrupt line (PCF8457). The items that made it all work for me, though in hind-site seem obvious, are:
1.) Never use a delay or any library which introduces a delay while in an interrupt (like the Wire library or LCD libraries). Serial communication, or any communication requiring a clock will also fail.
2.) Use an external pull-up on the interrupt line.
I was violating both of these rules under the assumption that the interrupt would have an internal pull-pull, and so as the interrupt line was "floating" it was causing it to sometimes trigger during execution, and then get stuck as I was attempting to use variuse communications in the interrupt code like accessing the LCD and/or set a debug LED with a delay, talking I2C/TWI, etc. in my interrupt code.
The fix then was to add the external pull-up to prevent it from firing incorrectly, and then to modify my code so that I only set a flag variable and do all my other processing in the main loop.
Thanks,
Josh