'87 CGT "overheating" after huge snowstorm

[Andrew Butitta]

  First of all, I'd like to extend a big thanks to Huw for identifying the radiator fan after-run relay. You're a big help, all of you. This list is an amazing resource for Audi fans!
   
  On to the problem at hand: My 1987 Coupe GT. The one with the deleted cruise control (among other problems)...
   
  So here's the story: the night before last, it snowed here. A lot. It kept snowing for 12 hours. I left work early yesterday after the worst of it (having to gun it in reverse to get out of my parking place).
  On my way home, on the highway, my coolant temp. gauge started flashing H (i have the digital dash) meaning engine overheat. Nothing I did seemed to make it go away (sometimes, decelerating or turning left will make the coolant gauge go back down to normal). It does this regularly when the coolant level is low, so I thought nothing of it and drove all the way home (parking in the snowbank known as our parking lot >.<  ).
   
  I filled up the coolant the next day and dug myself out of my 'parking place'. As I rounded the lot, I noticed that my coolant gauge (after 5 minutes of operation) was saying H, nearly overheating. My heater, on the other hand, was still blowing cold air, as it would with the engine only running for five minutes on a 12-degree-F morning.
   
  Long story short(er?) -- it was "overheating" all the way to work, although everything else was normal. Heat finally crept from the vents about 1/4 the way to work, like it normally does.
   
  What's going on here? In my mind, there are only two possibilities: the coolant overflow tank is finally giving up the ghost (as another lister predicted when I posted the pic of my engine compartment), or, the coolant temp sensor is dying, rapidly.
   
  The thermostat comes to mind immediately, except that I replaced it (brand new Audi/VW part) less than 4 months ago...
   
  So, I ask... What does the list think? Has anyone seen this before?