1988 Chrysler Le Baron Speedo inop occasionally

I'm having a random problem w/my speedometer. It sometimes works and at other times it seems to need to 'warm up' to work. I've replaced the speed sensor w/ a junkyard part's but got same result (random operation). The problem might be related to tranny fluid, as it began after I had the tranny fluid sucked out and replaced. Maybe too much fluid in tranny has foamed and is causing problem? Speedometer works about 2/3 of the time and after sitting sometimes doesn't start working for 5 or so miles. Or it will work for days, and then stop. It only has problem after start up, never failed while driving. What do you think? Thanx alot!