[Alibi]

I've had good luck with remanufactured ECM's offa Ebay...I've also had good luck with one from a wrecked car.

ECMs in general come with a new PROM chip, but it is a good thing to ask before you buy. Otherwise, all you have to do is remove the passenger side hush panel, unclip 3 electrical connectors, one 10mm bolt, slide it out, and repeat in reverse with the new one.

I had a Park Avenue with the same problem, it turned out that the ECM was bad (much like your problem sounds to be). Another possibility is that the electrical connections are just loose and the occasional jar knocks them back in long enough for the speedo to work for a bit.