a gps is an option... but would also be another distraction while driving...
after a quick google for "1988 BMW 325 coupe speedometer problems"
i found this stuff... for an 84 BMW 318i but it sounds very similar to your problem... has to do with bad solder joints on the speedo i think...
http://bimmer.roadfly.org/e30/messag...w47/31786.html
and
http://www.unofficialbmw.com/speedo/speedo.htm