UFO low and slow eastbound over I-40 between Flagstaff and Albuquerque

I am (then and now) a private pilot and (now) a Ph.D. physicist.  I was a student at Arizona State University and was driving home to Omaha after the Spring semester.  It was dusk (almost dark) as I went Eastbound on I-40.  I do not think I had crossed into New Mexico yet; I was not near any town at the time of the sighting.  I was driving a 1987 Pontiac Fiero (with the fuel-injected 6 cyl.) on cruise control at about 70 mph.  There were no other cars nearby.  Out of the corner of my right eye I caught sight of an airborne craft slowly overtaking me just off to the right side of the interstate.  The area where I was driving was quite flat.  I could not immediately identify the craft, but it had a rotating beacon underneath and a few other lights on it.  It was no more than 500 feet off the ground.
 My first thought was "this must be a cropduster following the highway to go home."  I kept switching my gaze between the craft and the road, trying to identify the craft.  I am quite good at identifying aircraft in the sky.  With each look, however, I became more intrigued because nothing looked right.  I could not make out position lights at the wing tips, and the rotating beacon (which cast light on the underside of the craft) was multile colors, including blue and white.  I could not make out the edges of the bottom side of the craft, though I got the impression it was bascially circular.  The lit portion of the underbelly appeared grey.  The craft was about the size of a modern-day jet fighter perhaps.
 Now very intrigued I sped up and caught closed the distance between me and the craft some.  My plan was to stop quickly, shut off my engine, hop out of the car, and listen for engine noise.  A radial engine, for example, sounds much different than a horizontally-opposed engine.  By the time I got out of my car the craft had proceeded away to a distance of less than 1/4 mile.  If it were a conventionally-powered aircraft of that size I could certainly have heard it.  The winds were calm on the ground.  I heard nothing.  Now I was very, very intrigued.  I jumped back in the car and gave chase along I-40.  I got up to about 100 mph and the craft appeared to be pacing me, not allowing me to approach it.  At that point I decided it was stupid to be going so fast and not paying attention to the road, so I began to slow down.  At that point the craft accelerated away and climbed.  The rate of acceleration was incredibly high.  The craft disappeared among the stars.
 The exact day is a best-guess.  Also, the year is not certain -- it could have been spring of 1988, 1989, or 1990.  However, I think it was 1989.
Witness indicates that the date of the sighting is approximate.  PD