Honestly I think it was the overheated oil that was steaming out. As I found there was pretty much no coolant at all when it finally stalled in my driveway.
Do you have an aftermarket lower radiator hose? If you don't have the spring inside the hose, the hose will suck shut at higher engine RPM's, but open when the water pump speed slows down. My '91 used to overheat on the highway after I changed that hose. The spring in the original was broken and I couldn't find another one. Learned my lesson and got one from the dealership.