I'm having trouble with my cooling, and it sounds like it may be a rust-filled radiator. When I bought the car the coolant was brown and contained rust flakes. I have replaced the fluid. The car stays cool (right in the center of the gauge) in town, but after a few minutes on the highway the temperature relatively quickly spikes to 3/4 to 5/6 up the temp gauge and doesn't come down, even at lower speeds. The thing is, when I give it gas with the cap off I see the fluid level drop pretty far. Doesn't that mean the system is circulating well?