1999 Nissan Maxima GLE Electrical system problems.

I bought a used a 1999 Maxima with a little over 160,000 miles on it. It seemed to run fine when I bought it, but I've started having some electrical issues. When shut off, battery voltage reads 12.8 or so. When the engine is running it reads as much as 19.25 volts. Is the voltage regulator in the alternator or in the car computer, and does it seem like that would be the problem? There are other electrical problems showing up, which makes me think that's the source of all the problems I'm having. One of the problems is that at low rpms (below 1000 rpm) the stereo speakers work fine, but at higher rpms they cut out completely. The stereo itself stays on, but no sound come out. The dashboard clock also comes on and goes off at random and keeps inaccurate time. The worst problem, which started only after I had been driving it for about 2 weeks is that the engine seems to be mistimed at most ropms as if the electronic timing advance wasn't working. Long question I know, but I want to make sure I'm not chasing my tail on this one.