2000 Durango weird electrical problems

I have a 2000 Durango,  first problem no power at
all, thought battery so took it out tested it seemed
fine so put back in started right up. So I changed
all hot and ground wires. Fine for a while then did it
again. Went to start at emissions place would not
turn over and the security light was on, jumped it
and it started. Then would peg the voltage gauge
for no reason while driving, pull over put in park for
30sec would go back to normal. Did that four times
in about a mile then just stopped. Has a new
alternator.