Alt. light flickering at high rpm???

I have this problem for the last few years
but never was that bad

Id replace the alternator for a 1 wire like this one

Hughes Engines

I went back to the shop that sold me this alternator asking if he could test it
it end up ok atleast that what he is telling me

with a multimeter if I connect to the alt stud and alt body it show 14.73 volt
and if I do the same test but ground the amp meter to the block and the other wire to the alt stud it show 15.... volt (that I dont understand) and its weird

instal #4 wire between the alternator and the relay starter
didnt help either
but the amp meter always showing disharge and to me its ok like that because most of the electricity go back to the batery by that wire and yes its still fleakering

replace the wire and terminal at the cowl dash
Under the dash and engine side
didnt help

replaced the batery ( FORGOT TO SAY IT IS IN THE TRUNK )
didnt help

instal new ground wire from the batery to the engine#1-0 size wire
and same size from the batery to the body of car

replace the pos. wire because I didnt instaled it and didnt know how it was so...
found a splice in that wire that wasnt right to me so Id replace it with # 2-0 size wire
from the batery to the starter relay
didnt help

all the terminals are copper and welded

WHAT THE HELL IS WRONG

at this point I think Im gonna let it burn to the ground and I will see where it was bad

please help me

forgot to say it happen at 2500 rpm and higher
also have ferd contour fan set up
first speed ON all the time and when I started the second speed
everything seem to be ok

needing more juice seem to fix the problem THATS EVEN WEIRDER