Random CELs (a misfire for example, which can be intermittent, say it's humid out or actually raining and your HT wires to the plugs are failing, they will short to ground when other times in dry conditions they will be fine) will clear with repetitive starts. Most cars it's around 40 starts or so, the CEL, which hasn't caused an error in the mean time, will clear itself. Built into the system.
Changing the boards may have re-set a poorly seated connector, or scraped corrosion off the connecting elements. It's a standard troubleshooting step, to re-seat any removable connectors, so no Voodoo involved in your experience.
Just recently I cured a non-working brake/signal/tail lamp issue by pulling the light fixture, cleaning the contacts, and re-installing it (I used continuity checks so I knew I had fixed the issue before I re-installed). Normal.
Everyone's power system is unique. It can be as simple as your neighbour (or even you or someone in your home) using X-10 devices, LED or CFL lamps on the same line as your HiFi, or some other noisy powerline contraption ... sending internet over power is possible; the neighbour's power leg is tied closely to yours from the distribution transformer, so you "see" his dirty power. All those Power Conditioners work to some extent, some better than others, but if your home system is fine from the breaker panel and your neighbours don't fall for junk plugged into the line, and your utility isn't some cheapskate cash grabbing scourge, they may make no difference at all.
