Motorcycle Repair: overcharge or spark, regulator rectifier, spark plug wire

Question
I have an 81 yamaha xs 400 when i start the bike it idles normal at 1200 rpm. I have a voltimeter attached to the positive batery terminal wire. The volts read between 12 to 14 but after a few seconds the volts jump up to 14 almost 15  then here comes the issue; the lights start to blink the same for the tach the rev arrow moves up and down little between 1100 and 1200 rpms and the sound of the engine change as it loses electric power. Note: RR gets hot!!!. I checked the volts with a digital voltimeter accelerating it between 4000 to 5000 rpms and it readed 14.7 to 14.8 volts.
Can it be an overcharge problem? I cleaned the RR, stator, and ignition unit connections and added dielectric grease. Also grounded the RR, to the frame but the problem continues. Also one piston stain the spark plug of black  carbon and the other not. I had changed several times that spark plug because of fouling. Could this be a problem with the overcharge or anything related to bad coil, old spark plug wire or cap, could this problem of overcharge and blinking originate spark problems or is more of old electrics?

Answer
Hi Rick.

 This could be either a short in the charging system wiring or the regulator/rectifier going bad.  Get the repair manual for the bike, if you haven't already done so, and perform all of the required tests on the regulator/rectifier as well as checking the charging system fo shorts.

Good luck and ride safe.
FALCON