amp meter needle is crazy

Ok lets Start at the beginning

Last summer I install a 1 wire alternator
You know the small one I think they are OE on Toyota
The guy who sold me this alternator told me it was a 75 amp
I taught it was enough for what I do
Msd
Fuel pump
Fan
Cd player
Amplifier
That's about it

This spring hade battery discharge problem 4amp drop
Went back to the shop he change something in it and call it good
Problem solve

BUt and its a but
Last week end went for a ride and came back home at night
I notice the heads like were blinking a little the dash light did the same
Took a look at the amp needle and it was bouncing

Ps I still use the amp meter but install a second wire from the alternator to the relay starter
Didn't wanna over load the amp meter and have fire or wire problem
I know I'm flying blind a little but the middle stay in the middle of the gage when
The engine running and show a little discharge when everything is running
Mad
Fan
Etch.

Please help me
Ain't the best with wire



Bob