90 K2500 Speedo Sometimes Works, Sometimes Not Usually Wrong

My big old pig just started this a week or so ago. Sometimes it works normal from startup but most times doesn't work at all. Other times it reads way low by 20 to 50 mph. The odometer works fine all the time. It is one of those arch type speedos is the best way I can describe it. If it is the cluster would a 88 Silverado K1500 direct replace it as I have an organ donor in the back field?