mysterious light follows car at a distance and shoots down into the valley

I was in 5th or 6th grade and we had just left a basketball game in Baker, Montana. The stretch of highway 12 west of Baker to Plevna, Montana is about 13 miles. I remember there was snow on the ground. The moon was bright and in the east as I remember. About 5 miles or so into the trip we were driving along and south of highway 12 about a half mile away or so was a light that if memory serves me correctly was round like the moon which I found odd because the moon was behind us. It "tipped over" on it's side and was oval and seemed to be going the same speed as the car and I told everyone to look and they did. It went along with us as far as the hill before descending into the valley where we lived. Then suddenly it took off at fast speed and went down into the valley out of our sight. We drove down toward our town and parked near the grain elevator on highway 12 and could see it out in a field about a half mile to a mile away. It was lit up and the whole thing went off and on (lit up off and on) a few times and then in a circle in the middle different color lights came on and flashed and it did this a few times and then nothing more so my dad said he would drive us home and he would go out on the road and see if he could see anything and that was it.
Witness indicates that the date of the event is approximate.  PD