40's chronograph service question

[vintagewaferthin]

I noticed that my 1940's Lemania chrono was running very fast(3 minutes or so) when it was sitting off of my wrist this evening. However, when I was wearing it, it was running much better, but was still a little off(about a minute give or take). What is causing such a fluctuation? Does body temp have to do with it when its on my wrist? Im assuming that the watch is definitely telling me its time for an overhaul, right? Thanks guys!