Sounds like an intermittent open circuit to me - a break in a trace, solder joint or lead that expands and makes contact when it is warm enough. Wild guess based on the behaviour you've reported. Intermittents are a bear to track down - if you have a hot air gun or hair dryer and a can of freeze spray, that can help you zone in on the general area, but in the end it'll be poring over the board with a good magnifier that'll show you the location of the fault - if that's what it is.