Loss of all power when starting my 2000 tahoe

Maybe I can get some advice here. I have a 2000 tahoe
with 260k miles.
Recently when I go to start it some times I turn the key to
start all the lights go off no click or any thing.
I have to go out and unplug my battery wait about 30
seconds and plug it in then it will start.
When I get it running the first Time after it dies my voltage
meter moves alot and at times can go from 14v to 12v or
less then jump up.
When I turn it off after this it dies completely and I have to
unplug and try again.
New battery less than 6 months old, new alternator went
from 105amp to 145 amp and that isn't more than 6
months old put a new starter in about 20k miles ago.
Any advice on what else to look for.