I changed to GoDaddy DNS lastnight as the problem was just unbelievably annoying.
ISPC/MYDNS controlled the DNS for a couple of years without fault. The problem started as soon as I moved to Google Apps for email.
Godaddy said it was a DNS issue on my end.
I copied each DNS entry from ISPC3 to the GoDaddy DNS Manager verbatim. It works fine now with their servers.
I truly think it was a caching problem somewhere on their end. But, I'm just wondering how I could diagnose MYDNS for errors in ZONE files to see for sure.
ISPC3 on Debian! It's great!