Radiator Leak: not warming up?

[daltron]

My wife drives a 2008 Aveo5 A/T and I recently noticed that the radiator has a leak on the passenger side (very top; facing engine bay) because A) it's losing coolant and B) I can see the rust from where it's dripping down the radiator and looks like always spraying the block.

The leak has gone from an occasional refill to the coolant overflow tank being empty multiple times a day. Let me say this now: I plan on replacing it in the next week or two (I have not determined if I want to try it myself or not). In the mean time, it's been acting strange though. For example, tonight we took it on a 70 mile round trip for dinner. Before we left I checked the coolant overflow tank and it was at the correct level (the car had been sitting since last night). When I drove off, I noticed that the temperature gauge on the freeway in no traffic was only about 10-15% (if C is 0 and H is 100), even after a good 10 miles. After we hit some traffic I noticed the gauge went up, however. When we resumed no traffic it went back down. Basically, heavy traffic roughly 50% normal operating temps, light traffic 25-35%, and no traffic 10-15%. After dinner (about 35 mile trip) i checked coolant tank and it was pretty much empty. Drove 1/4 mile to gas station, added water and some anti-freeze I had, drove 35 miles home. On way home same temp symptoms: traffic normal temps, no traffic extremely low temps. We arrived home and I immediately opened the coolant cap with a glove. Very little pressure but levels seemed OK but I could see/hear the radiator leak.

Like I said, I plan on replacing the radiator soon, but any thoughts why the car wouldn't stay warm when at highway speeds but would come up to operating temps when in traffic and the back down to cold temps when traffic ceased?