Which component can cause memtest86+ wrongly detect defective RAM on MacBook Pro A1286?

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Peperonix

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I have here a MacBook Pro 15" A1286 EMC 2535 that is showing strange symptoms :
  • works like a charm and could be fully reinstalled, but sometimes shuts down abruptly after a few minutes.
  • memtest86+ run from Gparted live CD fails with any RAM stick, whatever the brand (Samsung, Hynix, ...), the speed 12800s, 10600s, 8500s.
  • attempting hardware diagnostic through Internet Recovery (Alt+D at start) also fails : the logo shows the earth that rotates during 3 seconds, and then stops rotating.
The paradox, is that in this MacBook, memtest86+ reports defective RAM from the very first bytes, whilst running memtest86+ from the same Gparted live CD on a 17" MacBook of the same generation (A1297), the RAM sticks can be checked as OK.

The computer has Intel HD Graphics and a dedicated NVIDIA GeForce GT 330M, the later being listed in the system report (but not in the "About this Mac" summary).

I assume memtest86+ detects some defective RAM that comes prior to the RAM sticks.
Not sure if the RAM chips soldered on the motherboard are used for that. (N.B. They are likely the graphical RAM for the NVIDIA, as seem soldered opposite to it.)

The purpose of this question is not whether it is profitable or not to diagnose/repair such old laptop, but getting a better understanding about what can make the computer crash. The SSD itself is OK and the file system has been checked.
 
MacBooks from that era can have an issue with the soldering of the RAM carrier to the logic board. Sometimes only one slot will be flaky, the other just fine.
 
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