98 Explorer Sport Intermittent Speedometer Problems

The speedo in my 4X2, 5-speed, 4.0 pushrod 98 Explorer Sport has
started to work only about half the time.

On a 1,000 mile round trip this weekend, it worked for less than 500
miles. On the way down, it would work for around 100 miles, then not
work for about a 100. On the way back it worked for the first 250
miles and then didn't for the second.

There doesn't seem to be any rhyme or reason to when it will work or
when it won't. About the only sure way I've found to make it work is
to take it to a shop to have them check it out.

Computer checks have ruled out the speed sensor.

Now it's either the wires, the head or the computer, but the only way
to figure out which seems to be to replace one and see if it fixes the
problem, which could be really expensive.

Anybody have any experience with this problem? Any advice?