Temperature question

[Grandavi]

Not sure if anyone has experienced this before, but here goes...

Last year I had my rad pressure tested and everything checks out find except I have a tiny leak in my heater core (they mark that as 9 hours for remove/install). I am about to have that fixed and then switching from regular "green" coolant to the "red" coolant.

However, today I took the car out for a couple hour cruise with the targa off. On the highway, I was between a Prelude and a Hyundai (tiberon?) and we all just kind of went for it (3 lanes of fun). I had the car ahead of them so I just held off a bit and kept slightly in front of them at 185 kph. After about 3-5 minutes of that, i dropped off and went down to 130 kph. Thats when I looked at the temp gauge. It was right below the red tag at the top of the gauge!

I slowed down to 100 kph (62 mph) in 5th gear at about 2600 RPM and the temp went down to slightly above normal. Drove for about 10 minutes, then pulled over and looked over hoses, the reservoir, etc.. and everything was dry and looked full (although slightly overfull in the reservoir).

What would cause that to jump so damn high? If I keep the car at 120 kph or below 3500 RPM, the temp stays normal. When I go higher, the temp starts to climb. No knocks, rattles/pings, etc.. everything seems perfect, although the oil is down about 1/2 a litre (quart).

ANy ideas? I am wondering if the thermostat did something wonky or the water pump is seizing under high load. (although I would expect a noise)
No warning lights, oil pressure was steady.