A strange phenomenon

Perhaps it is an electrical supply problem to that outlet or the panel.
There is a surge and then a lag and the amp cannot compete with the irregularity. Personally I would be concerned with a "short" or some other
safety/fire hazard issue and bring an electrician. Last question, is there a periodic large power draw, like a well pump etc. Not an electrician but have
experienced dimming lights for no reasons etc. Lastly, I wonder if the
voltage regulators, capacitors or similar devices within the amp are functioning correctly.

Maybe after a few hours of listening you just get tired or bored. Your mental state could definetly affect your perception of the sound
Alan

If there is no technical problem such as something with your ac or one of your components, then it could simply mean that you have a poorly matched system that once warmed up just doesn't sound good. A system's sound will change when it goes from being turned off and cold to being on for a while and warm. Some never turn off a system because of the warm up process unless they have a system with a tube amplifier. You do not provide any information on your system. It could be the basic issue of amp-speaker matching (the foundation of any good system, imo). Putting together a good-sounding audio system with a separate amp, preamp, sources and speakers can be a real challenge. It requires research, auditioning, listening, trial and error, as well as simple luck sometimes. You can hit and you can miss.

Yes, Mr. Fishing's thread was entertaining with 500 comments, yet very frustrating. I also recall he had a wire incorrectly hooked up.
The most likely cause of his system sounding dynamic, then sounding anemic after warmup was a mismatch between amp and spkrs. His speakers had an impedance curve which went as low as 2 Ohms and he did not have an amp with enough current to drive them. I know he is still auditioning new amps with higher current.

Musicland1, what amp and spkrs are you using?

I tend to agree with what Arh said. Systems that have that initial "wow" factor tend to wear on the listener in the long run. You may be mistaking what you hear for listener fatigue. Symptoms can be as you described, the desire to do something else besides listen to music.

Shakey