Switching RAM modules fixes boot hiccups. Why?

Question

I have a rather old motherboard with 4 DDR2 slots, all occupied.

I bought the board, RAM modules, and CPU from an acquaintance, second hand, so I do not know the exact history of the problem.

The problem is:

On rare occasions, the machine fails to boot. i.e. When I press the power button:

1. I see no display at all
2. The CPU fan runs, stops, runs.. repeatedly with the same frequency.
3. The Speaker does not make the usual on-boot 'beep'

When I asked the original owner about this problem, he asked me to 'pull out the RAM modules and switch their slots'.

I was sceptical about this solution, but I tried it, and the machine booted.

Since then, whenever this occurs, I have found a cause-effect relationship between 'switching the modules' and 'the machine succeeding in booting'.

My question is: Why does switching the modules (temporarily) fix the failure to boot? It does not seem logical. I guess the answer would also need to explain why this can happen in the first place.

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If it matters: The modules sit flush in the slots (they are pushed in properly). They are Corsair XMS2 modules of 1GB each, the heatsinks are thick enough that they (almost) touch each other when on the board.

Answer 1

Since the mainboard and RAM are old, there might be the possibility that the connector is dirty (oxidized).

Why removing and re-inserting the modules 'fixes' it

The friction generated when re-inserting the modules probably scrapes off the oxidized material, thus providing better electrical connectivity.

How to verify this

Next time the problem occurs, try removing and re-inserting the modules in the same slot. This should work as well.

Or even better, clean the contacts on RAM with contact cleaner.