And where do you note this from? The cable in your home, sure, it's solid core if you want it to be - though I prefer 3x1.5 or 3x2.5mm. There is no 5 or 8mm solid core domestic mains wire anybody makes, and if they did it wouldn't bend through the walls. If you could pull out your calculator, you'd see a 5mm radius wire will take about 8 inches to bend 90 degrees without a kink, and last I remember they didn't make holes in walls that wide. Specially not in the US (the walls there are less than 6 inches thick), and not even in India - the walls are max 10 inches thick, and not even that sometimes.
Power company cables are usually never solid core, they are multistrand for sure. One strand is 4mm or 6mm, but it is stranded. Have you heard of thermal fatigue? If power cables were indeed pure solid core, they would crack apart in a few years. Stranding is necessary for thermal relief. Just like a railway line is not a continuous line. You'd see this if you were a pigeon crapping from one, though from the ground it does look like solid core. Be assured it is not.
As for power cords, just because you hear what you want to hear (most high-end audio is placebo, beyond a point) does not mean everyone will. Your opinion is just that, an opinion. I don't see why I should believe it as gospel truth, number of posts aside. Your ears are not proof enough for me.
The fact is that power cable does make a difference. Without it, systems will be totally silent. But if it presents a low enough impedance to the mains, its job is done. More than the cable itself, the quality of the contacts and mating surfaces may cause problems with high frequency arcing, and it is recommended to keep them clean and free of oxidation. Which is why brass is used for these connections, as it resists oxidation and is self-cleaning on mechanical insertion. The other metal that does this is silver, and that is the only reason for preferring it, not because of some snake oil audiophile property.
And FYI, most Cardas wire, for example (sold by Percy) is actually multicore, golden ratio wire - I have quite a stock of it (I'm in India). You need to be a little more thorough in your research. I have a copy of the Percy catalog, I can't find any solid core wire suitable for a mains application, maybe you would be kind enough to point out one? Apart, of course, from Goertz, which cannot/should not be used as mains cable. And which connectors? Be aware that the Audioquest mains cables are multistrand too, except the strands are individually insulated and are thick - I guess 1.5 or 2mm each. It is still multistranded, though. The interconnect wires are pure solid core.
Also, a solid core wire will have lower capacitance, but higher inductance than an equivalent multicore wire. There is no magic bullet, and a heavier guage of wire and better connectors may be all that is required to ensure the mains into the equipment is clean enough. The cable assembly can do no more than this, so if you are experiencing an improvement by switching cables, it's likely your earlier cable was junk (you'd be surprised at how many of them are actually junk, specially the free ones).
Or, more likely, you are hearing the dollars leaving your pocket. This is not meant to be derisive. It is a real phenomenon, and happens to the best of us. Experimenter expectancy led Neils Bohr awry, we are but mere mortals. As long as you are happy, it's all good. Just don't expect people to believe you because you say so.
And yeah, get a spellchecker. It is part of the Firefox browser. It looks ridiculous that you would indulge in name calling when you can't even care to spell right.