[VR43000GT]

Hmmm, thats tough. I would assume that you're not overheating at highway speeds because there is more airflow to the radiatior. Do you have enough anti freeze and is your water to anti freeze ratio in check? You may need to put some more anti freeze in because if you have too much water it may have trouble keeping cool at idle with no wind flow coming into it and it could keep cool at highway speeds because it has more air flow. Good luck.