Variations in how your system sounds?

Apologies if this has been covered elsewhere.
My system sounds consistently good to my ears for the majority of listening sessions but every now and again (say 1 in 20) it sounds slightly 'off' -- not its usual self. It is a very small difference but i do hear it. I would describe it as a slight lack of focus and dynamics. This has happened through many changes of all components.
Is it my mood, or hearing variation? Do i need to filter my power? Do you have similar experiences?
You didn't mention if the problem shows up with all sources , that may tell you allot . I used to have an issue with music sounding better in the evening , I upgraded to a high end conditioner and that problem was solved .
If I am extremely stressed or fatigued, my system just doesn't sound right - as you describe unfocused with flat dynamics. Over several years, I'd experienced this with frustrating irregularity. Then I discovered that when I ingest a B-complex multivitamin the night before, my system always sound oh so right, glorious, the following evening. I have reproduced this phenomenon enough times that I cam convinced of the causality. From a medical standpoint (I am a physician) a logical explanation can be made, having to do with neurotransmitter depletion and general neuronal fatigue. As an aside,I have completely foresworn any alcohol in my diet for 5 years now; for me, alcohol consistently smears and thoroughly ruins the critical listening experience, even in small doses.
'for me, alcohol consistently smears and thoroughly ruins the critical listening experience, even in small doses.'

I have found the opposite to be true in my case. A couple of shots of I.W. Harper, and my speakers sound like the Wilson Maxx.
Rok2id...from earlier in the thread: "I recently bought a Hi-Fi Choice magazine with a free System Set-Up disc." That's how. I'm here to help.

I have to warn the doctor that by "foreswearing alcohol" the girls will not get prettier at closing time, and telling 'em you're into B vitamins...well...that just never works.
An interesting subject and I'm sure it is a true observation. I am not convinced it is physiological, I'm a physician too. Some days, you turn the system on and it just is'nt firing on all cylinders and yes, I do give it time to warm up. I am not one of the leave it on all the time, believers.
I am not ware it is connected to how I feel, tiredness, anxiety levels. I have a pretty good power supply, with a Pure power regenerator. So I can not explain it, but I agree with Mike60's observation.