Thanks for the input, folks.. Here's where I am (and I'm open to debate on these conclusions):
1. The fan clutch *seems* to funtion properly - does not freewheel at all, hot or cold - can usually spin maybe 1/4 turn, actually, not even.
2. The radiator is warm/hot all over at operating temp, so I don't believe it is misbehaving.
3. The fact that turning on the heater brings the temp down so fast leads me to think that the thermostat is acting right.
I'm left thinking that there's got to be a problem with the auxiliary fans. I don't guess this could be as simple/cheap as the pressure cap...? I'm also wondering if the cool harness would make a difference here, given the situation. Remember, outside temperature was in the 40's or 50's last time this really occurred, also the last time I was in in real traffic. I've been in some lighter traffic the last few days, and seen it reach almost 100 C, but nothing too exciting. I still think that it should reach an operating temp and stay there, and I've read differing opinions on this.
I will check out the fans, fuses, and relays this weekend. Any other thoughts?
Again, thanks for the replies.