"Tom, quick question- did you at any time during your "problem" runs get thirsty or want to take a drink?"
Yes, and I drank, but not enough apparently. They put two liters of electrolyte solution in me after the first marathon and told me I fortunate to have strong kidneys. I suspect part of the problem was that I trained for those "hot" marathons, which were run in early May down around Lone Pine, CA, up here in Seattle. Thus, I was not acclimated for running in heat for that length of time. Altitude and the arid climate may also have contributed to the problem, IMO, and time on feet definitely did. I ran the last half of those marathons when the temperatures were in the mid 80's to low 90's. The 50 miler was also run in fairly high temperatures, in the low 80's for the last 3 or so hours. In truth, I wasn't adequately trained for the 50 miler. I was very marathon fit, having come off a 2:45 effort 3 months previously and cycled up again to marathon racing fitness, but was simply not trained to spend 8-9 hours on my feet in hot weather. That marathon required no fluid intake, whereas an 8-9 hour effort in hot weather definitely did, and I hadn't trained my body to process the water efficiently, or so it seems to me. Ultras are a whole different ball game.