One thing that I ran into during my years of servicing torpedoes and computers was random failures due to "flex" and/or "thermal" faults. Theses are faults that show up when a part it flexed or when it changes temperature. The most common source of the problem would be bad (or "cold") solder joints, but sometimes it was a physical crack in a component.
but could heat be affecting something else that I can't easily measure?
In a cold solder joint, the connection may look normal (shiny) or it may look dull, but the main thing is that there may be no actual solder connection between the parts - say, a wire and the circuit board - and the parts are essentially just touching each other. Corrosion can get in and make matters worse.
Anyway, as the parts go from ambient temperature to operating temperature, they can shrink, or expand or flex relative to each other and this can lead to a loss of electrical connection, often in a very random manner.
We would often track down thermals by using "freeze mist" to cool suspected components, This can also basically be done by using a "duster" upside down.
Long story, short - heat can affect components even within normal operating temperatures, so temperature readings often show nothing unusual.