Electrical glitch

[Mougoo]

Hello every one.I have a gremlin that I cannot seem to kill.
I have an 1986 300e with 170000 km on it.
I was having current draw problems and that were draining my battery.With the help of the forum I found out that my car alarm was killing it so I removed it.
My car has a new battery 1050 CCA and a new alternator thats putting out 160 amps.(custom built).But when I drive the car at night with the lights,heater, on it will loose power and eventually shut down.
I tested the battery at the shop along with the alternator my readings were 12.75 v at the battery when the car was off.
and 14.2 when the car is running at idle.
My abs light would come on intermittently but I know that the ovp is fine cause I installed a new one .My ABS is working because when I hammer the brakes it kicks in.
All my battery leads and alternator harness are new as well.
I have trouble shot as much as I could but now I am stuck.
Sorry for the long post and I thank you in advance for the help.