I have the same problem with my system. When it's good, it's magical. When bad, unlistenable. And it will change on a moments notice and back again. The entire bass spectrum will disappear and the top will get gritty and grainy. And it's not related to any particular component as it happens in 2 different rooms with a multitude of equipment
I looked into my house wiring, built in 58', and it's obvious that dedicated lines are a must. Typical home wiring consists of a few to a dozen or more splices involving twisting 2 copper wires together and/or push in connections that are nothing more than the edge of a piece of brass touching one side of the wire. In my home, they actually used aluminum pieces to crimp the 2 wires together.
Needless to say, the line resistance varies with the humidity, temp, and power used. Copper corrodes rather quickly and you can imagine what it's like after 30+ years.
I would not think of auditioning any power conditioning/power cords untill you have installed dedicated lines. Use 12ga romex and put one twist in the wire every foot or so to make it less of an antenna.
One other thing that I know effects my system is the power down of the AM radio stations at sunset. The radio stations are required to reduce power something like 90%.You can actually hear a layer of grundge dissappear about the same time each night. I initially thought this had to do with EMI from the sun lessening it's effects on the power lines after sunset. But it was way too abrupt-like flipping a switch and the time changed about a minute each night-tracking the daily change in the sunset.
In general, dedicated lines are less expensive than power conditioning and have a more positive effect.