Overheating

I have a 1996 Hyundai Accent GT 16 valve five-speed manual transmission with 153K miles. The car over heats when traveling faster than 65 MPH. I don't see a lot of movement when I open the radiator cap, when hot and no foam, and I don't hear any noises from water pump. The car stays cool traveling a low speeds (all day long), but I removed the thermostat and have noticed a slight improvement at high speeds. It should be noted that the car has been extremely hot a couple of times and I must top-off anti-freeze (about a pint) every two days (I travel 200 miles per day). Spark Plugs have shown a slight green tinge at tip, but overall the car runs great (especially in cool weather). Both radiator fans work and there is no leak anywhere I can see. I once put in some fiberglass stop-leak, maybe too-much?