Please post back if you find a culprit!

I'm still battling this on my 89, and have so far tried running a new set of wires to the sensor from inside and cleaning all the contacts like you did. The speedo looks fine, and when it happens both speedo and odo stop working.

So if the replacement works, please post.

You may also find that the OBD reports no speedo signal, which is because the speedo is what reports it.

With my issue, sometimes banging the dash works, sometimes it doesn't, and it goes on and off without much provocation, I'm just driving along and it suddenly works.

I have noticed that it works more now that it's warm outside.