Originally it was fine around town, could drive it anywhere without overheating (gauge would stay at about a quarter). As soon as i took it on the highway with high revs for 30mins + it would overheat. Took it too a mechanic, he replaced the radiator with one that was apparently larger then the last, and he said the last was 60% clogged. Now, driving around ANYWHERE (only done about 10 minute trips so far) the heat is staying about half way and when i stop i can hear the water physically boiling. Took it back to the mechanic, explained it, he replaced the "bypass hose" and did something to the thermostat housing (not sure).. problem still remains however.

any ideas?