Hey I'm new here, but I had a similar problem that had a different source obergc and I spent a fortune finding out.
The speedo would work at times then randomly plunge to zero. Then sometimes it would jump back to my current speed, and sometimes it would hit 160(going 80mph). From what I hear this is fairly common. . . I think especially since it is intermittent it more than likely is the same problem. I had a Lexus dealer *sigh* *wasted money* diagnose the problem and it turns out the speed sensor in one of the wheels was faulty. I believe there are three total, one on the tranny and one on each wheel? Anyway I had a seperate import shop swap the speed sensor for a new one and the problem was fixed. $90 for the Lexus inspection, and if I recall(I did this almost half a year ago) the job costed $240.
1990 LS 400
