Since the really bad spiking, we eliminated the suge tank as I said before, and ran a straight fill tube. I think once it crept up to boiling point (we didn't have a full system, so this was happening pretty easily), we'd lose even more, get vapor pockets, and the system would not flow. There were times our radiator was ice cold yet the temp gauge read well in excess off 100C.
The spiking has stopped, we're not losing coolant, the fans are kicking on when they should be, etc... After each drive, we've found the coolant level "drops" slightly, but there is nothing in the catch can, which implies to me we're getting air out. The last test drive was spent redlining in gears 1-3 and a lot of very spirited driving. It didn't overheat that time, but crept up over 90C (195F). Temp was slowly up and down in the 70-80 range (stat opening and closing) and was climbing slowly up over 90.
At this point, things appear to be functioning properly (no major spikes) but at the high end of where we want to be. The plans right now are to replace the auto zone thermostat with a Robertshaw high volume unit (with bleed holes), to replace our underdrive pulley with an overdrive one, and to replace the 50/50 water/
antifreeze with distilled water and water wetter.
I am hoping this will get our operating temps more consistently in the 170-180 range, seeing 190-200 only under extreme driving, in bad traffic, etc...