<VV> "partial" thermostat failure?

[Derek Archer]

I have a '64 spyder coupe and noticed that while driving, the 
temperature guage was reading a lower temperature than it usually does, 
so my instincts tell me a failed damper door thermostat was to blame. 
Sure enough, when I stopped, the left side one was wide open and the 
right one was barely open at all. This explained the lower guage 
reading as the thermister is on the left side. But when I got home and 
let the engine cool off a while, the door closed! This puzzled me, so I 
started the engine and let it run and the left damper door opened but 
not the right one. the gauge was reading about 300 degrees or so when 
it was at full open and the right one had only barely begun to open. 
Basically, the left one is opening way too early or the right one is 
opening too late (not likely but possible). Should I replace just the 
left thermostat or both of them?

Derek Archer