ok so i took a couple videos, they can be found here:
http://www.youtube.com/user/grandpasgrandson?feature=mhum
The first video i took was the "Thunderbird Test" and in this the tach works like it is supposed to.
Then in the next video on the way home it acts up and to my luck i caught it on camera lol. any reasons why itd stall out like that?
P.S. Yes yes i know my windshield and dash are cracked to hell. lol
Have you checked your wiring and connections, especially your ground wire? I would start there.