Hi, I have a 1991 300zx, non-turbo automatic. When I'm cruising down highway, in either 4th or O/Drive, it'll just drop a gear by itself. Once accelerating a bit and backing off, car will go back into correct gear for speed. Not long after it will drop a gear down again. Has anyone experienced anything similar and found what fault it is? I will be servicing transmission in the next week or so but thought I'd put feelers out to see if there is anything else I should look at as well.