It really sounds like some part is ceasing to function once it warms up. Since it affects both (stereo) channels, this means it pretty much has to be a component that is common to both, which rules out a lot of stuff.

I supppose the DSP chip might have developed a bad solder joint somehow, possibly on its clock circuitry. One way to test for that, might be to use a small bit of programming code that reads known-good values back from the DSP. If the chip is dead, those values will be bad.

This might best be done from the Hijack menu, but no such code is there today. I'll think about it, but no promises.

Cheers