UPDATE:
Well, I think we might be close to the problem. To refresh everyone's memory: the overheating (1995 Z28) came on suddenly. I did the following in an attempt to remedy the issue (listed in order of replacement):
- Replaced thermostat with a Stant SuperStat heavy duty unit
- Replaced radiator cap and flushed system, using Delco-spec antifreeze
- Replaced water pump with brand new AC Delco unit
After these replacements, I would start the car and upon warm-up, the temp gauge would continue to climb into the red. As a reminder, this water pump is shaft-driven from a gear off in the timing cover. Look here: