I spoke to George over at Beemers Uber Alles this evening. He suggested it may simply be a problem with the rad cap. Reasoning that the coolant gets hot, expands into the catch tank, and is possibly getting caught (no pun) there unable to get drawn back in to the system when cool. The cap looks OK and I have it soaking for now. I'll try putting everything back together tomorrow PM, bleed the air out of the system, and test it close to home. George's also thinks there is the possibility of a bad water pump. I sure hope not. Not looking forward to having the bike down for so long.
"An expert is someone who has made every mistake possible within a very narrow field of inquiry"