Relay stuck - radiator fan, 86 4kq

["Dave C." <conner@cfm.ohio-state.edu>]

The radiator fan occasionally continues to run on after the temperature
drops to the point where it should stop.  I can make it stop by tapping on
the relay.  There are three identical relays in the fuse block and I've
swapped them all with the fan relay and it has happenned with all of them.
The other two are for the A/C and the horn.  This doesn't happen every time
but it's too often to ignore.  
What is causing this?  All these relays are probably old but they work OK
for the horn, and the A/C has been removed so I can't say how that
functioned.  Would a shiny new relay fix the problem or is something else
at fault?  
Ideas anyone?

Dave Conner
Columbus, OH
91 200Q 20v avant 
89 100E
87 5KS 
86 4KQ