86 5kcs radiator fan problems

["Daniel R. LeFebvre" <danbu99@bu.edu>]

hello.  the radiator fan has stopped functioning properly on my 86
5kcs.  it has only one speed, ultra fast.  this is a probelm living in
the city, in that the engine will become quite hot, and then the fan
will kick on.  hooking up a voltmeter, i realized that when the fan
comes on, the voltage drops incredibly low (below 12 volts).  if the
headlights are on, they blink when the fan comes on.  also, the fan will
not come on to cool the a.c. condenser, so a.c. does not function well
unless i'm traveling at speed on the highway.  i popped in a new relay
just in case, but there was no change.  is this a common problem and is
it easy to fix?  thanks!
dan