Dash lights black out and charging problem
 

I have a 1992 LS 400 which i have owned for about 6 months. I have had various electrical problems and resolved many with the fix of the wire harness at the trunk hinge. Two problems still remain. My dash lights periodically go black and then will come back on. Also my charging system is not keeping my battery charged some times. Since i bought it i have had the dead battery problem randomly on occasion. I first took it to the dealer and they replaced the Alternator and The battery. I have volt meter displaying voltage when i am running and much of the time it will cary around 13.3 to 13.4 volts. Other times it will drop to near 12 and there seams to be some correlation to when the dash lights have blacked out. Does anyone have any thoughts?