dieselsrock

I havea '96 Ford Aspire w/ 72K on it, unfortunately the speedometer is acting up. Recently, the speedometer has decided that while accelerating, the needle moves down towards zero, but when you ease up on the gas it rises back up. Once you reach your desired speed and are no longer accelerating, it seems to level out (still minor fluctuations), and you can see how fast you are going. I was wondering, is this the start of a terminal condition for it? IF so I was goin to see if I could get an instrument cluster with a tach in it. Would that be a simple unplug and plug back in to get the tach to operate or not that easy? Appreciate any expertise you all can offer. Thanks.