[vgames33]

I don't know why the gauge would only act up in a certain temperature range, but if jumping it helps then there might be a short inside the gauge or something. You might want to try replacing the gauge.

When you hit the brakes, I don't think the voltage should taper off, but 14 volts doesn't sound that bad. How old is your wiring? Is the alternator old? Are there alot of aftermarket electrical parts?