Okay, I know that technically I'm cheating a little here, as my Sub is a 1990, but I'm hoping that some of you may have enough overlap to help me out on this...

I have an "electronic" speedo, which uses a tone ring instead of a gear in the tranny tailhousing. Lately, the needle has been wandering at times, over a 30 - 40 mph range. After a while it settles back down, but in the meantime I'm SOL if I want to make sure I'm under the limit in residential neighborhoods, etc. Has anyone had any experience with these? Any suggestions where to start looking for the problem?