I've had this problem recently too with the exact same symptoms you describe; only on cold starts and only above 25-30 mph. I was going to write a post but the problem went away with warmer weather and when the vehicle warmed up. I assumed it had to do with cold gear grease in the speedo assembly as it sounded like the plasitc teeth were grinding on each other. I'm actually happy some one decided to create a post.

1989 Montero 2-Dr 3.0L V6 15X8 ProComp 97 Series w 30.5x9.5 Cooper M/T's

http://www.cardomain.com/ride/3143932