HI! I'm new to the forum though I have read many post on here, I'm experiencing a odd issue, and any help is appreciated! My speedo works fine when I start driving, after 10 min on the road, the speedo gradually drops to 0 with the car moving normal speed. If I stop and let the car cool off, it will work again for another 10 min. It happens like clockwork every time I drive it, works fine for about 10 min. Also it knocks out the OD, and or cruise control when it drops off! and sometimes I get a CEL (not always) code 24(VSS). Gage cluster was rebuild not long ago, no change in issue, Tried a cluster out of another car...No change! also replaced the VSS...No change! Car is a 1988 with the DNE 4+3 engine rebuilt to +.30 mild cam, intake and exhaust mods, drivetrain stock. 

Check the VSS wiring for chaffing &/or heat/melting damage and/or grounding.  Also, check all of the VSS connections.  I'm thinking a small detail of poorly positioned wiring during the car's modification process caused wiring damage.  

Thanks for the response rtslt4! So far no one I have talked to has ever run into this issue! And it is hard to test for since it only happens when the car is hot! But I am on the same page with you...Thinking it is wiring or the drive gear itself is spinning once it gets hot and expands a bit! the problem definitely seem to be heat related! and most of the other causes have already been checked!  I have checked all the wiring I can get to and it looks fine , but there is a section under the distributor I cant get to without removing it, that is next along with checking the drive gear, I need to drop my exhaust to get a better look! I purchased the car as is...Beautiful clean car with a bunch of little bugs the previous owner just couldn't figure out! I have been...a little at a time fixing them! If all else fails, I may be looking at a trans swap!