Sometimes my system sounds great, sometimes awful

the poster who mentioned the power coming out of the wall is probably right on. I lived in an area with a stressed power grid, and I was shocked at the improvment I heard when I added a good conditioner with battery back up. Made a huge difference, more than anything else, tweak wise, I have ever tried.

I've read mixed opinions on conditioners, some say they kill dynamics, probably not all of them.
What I notice the most is how sometimes everything sounds flat and muddy, then sometimes I wonder how everything's sweet and engaging. Maybe I have bi-polar ears.

I experience the phenomenon often. I think Macdadtexas (and the poster to whom he referred) has the best single answer in terms of gear. Not only are there different power draws throughout the grid at different times of day, but there is a different level of electronic interference in the air (including household) and in the grid. If I had to pick times when my gear (generally) sounded the best, it would be early in the day--say 7:30-8:00 a.m., and a couple of hours after dark. So, even though I don't have one, a power conditioner seems like the one thing that could level off SOME of the inconsistencies. I've been eyeing the newest Shunyata conditioners (especially the Triton).

The other thing to blame is simply our moods. Plenty of people have alluded to that. Kind of like a biorhythm thing. In my scenario both of the timeframes I mention are either before the tension of the day is built up (a.m.) or when I have escaped it at night. Sometimes my system sounds the worst when I am trying to use music to get away from the BS/stress. Of course, nothing has changed about the gear--it's all in my head (stress, etc). There are times I just cannot get real joy out of my gear and some of those times I even blame the gear! It's idiotic, of course. I'm sure that even after I manage to assemble some sort of undefined "ultimate system" (for my means, anyway)I'll still have plenty of moments where the music is still not totally in synch with my head. It's those moments when it IS that I treasure.

In addition to the incoming power do you have a dedicated circuit for your system? Not sure if that would help but when I added 2 for my system the noise floor dropped significantly. That being said my incoming voltage varies from 119V to 127V and I hear no differences at either extreme. Before I knew my voltage was all over the place I added a 2.4KVA balanced isolation transformer that lowered the noise floor even more. Since then I have taken it out because there was a 2V gain on the output and at 127v I had 129V out. Can't be good for my tubes. Personally when funds permit I'm going to look into voltage regulators.

"Do we hear differently depending the time of day, our mood, heat, cold... other circumstances?"

You left out recent noise exposure, current condition of sinus's, current emotional bias (I just changed this thing so now I'm going to believe music sounds better or worse), social pressure, humidity, ego, and probably lots of other stuff I'm not considering.

On the bright side, your knowing about all this stuff ("tell myself to wait until the next day") can save you a boatload of cash over the long haul and keep you from stepping backwards now and then.