These speedo's and odo's are mechanically driven. You can determine if it is actually a speedometer component that is malfunctioning if you can check the opperation of your odometer. If the odometer works correctly and accurately when the speedometer won't work, then you know it is a speedometer problem. If not, it is the drive for the entire instrument. Be aware that the older odometers tend to slip when they get old.

When you determine what the problem is, post back. However, I'd presume that the problem (if temp sensative) is caused by the mechanical cable that transfers the wheel's turning force to the speedo. This can bind when old and dry of lubricant. It is most likely the problem, as the speedo itself isn't servicible. The cable will probally have to be replaced.

Sincerely,
ATLD