Another overheating thread

[Black88GTA]

I was out driving today, (about 50 degrees, sunny) and got stuck in traffic. Everything seemed OK, until I looked at the temp gauge, and it was at 3/4 of the way up So I pulled over as fast as I could, and took a look at things. Here's what I found...

Both fans were on, and working normally.
Overflow tank was beyond the full mark, but not bubbling / boiling or anything. In fact, I dipped my finger in it, and it was barely warm.
Radiator cap / top of radiator felt warm (not hot)
I could put my hand on the intake manifold with the car running and leave it there (warm, not hot)
Upper radiator hose was warm
Oil on dipstick / under cap looked fine
No leaks at all.

So, I allowed it to cool down for a few minutes, then decided to turn back and drive it home (with an eye on the temp gauge, and the heat on full). I noticed that when I was idling / moving slowly, the temp would go up. But at expressway speeds, it dropped to normal operating temp and stayed there (never went below normal). The car drove fine - I wouldn't have even known anything was amiss unless I looked at the temp gauge.

The radiator was replaced last summer with a new one. Water pump / thermostat are unknown age / mileage. Although I would guess that the thermostat is probably OK, since the temp never went below normal when I was on the expressway.

Does anyone know what this might be? I'm suspecting either the head gasket, or water pump. God I hope it isn't the head gasket. I have no idea how I'm going to fix that here if it is.