Hey Russ, it's the Hijacker here. Messed around with my Neon the better part of Saturday. New speed sensor=NO change.
Started tearing into gauge cluster. Poor connection between cluster and dash wiring. Gave terminals a little twist to improve connection. Result: still intermittent at best. Built up terminals with a little solder. Same result!
Maybe there is a break in the wiring somewhere. I haven't found it yet! I feel your frustration with this. We probably put 10-15 miles on the car, on the hoist trying to find the "sweet" spot where the speedo would run consistently.
If we ever trace it down, I'll give you a heads up.