A possible culprit could be your O2 sensor (Oxygen sensor in the exhaust manifod or piping). These are usually good for about 80K to 100K miles. When they fail or start to deviate too far from their baseline readings they can cause the check engine light to come on. Driving on the highway at higher speeds would result in higher exhaust temperatures than typically are encountered during in- town driving so this could explain why it only occurs on at higher speeds (which equal higher engine loads of course). You could buy an engine code checker or take it to a shop and have them read the error codes for you to narrow it down and/or confirm my O2 sensor idea.