Error in RAM Integrity Check after driving my MacBookPro around in my backpack?!

I know the topic sounds strange but that's basically what's happening to me..

Short summary:

I own an early 2011 MacBookPro 15" (MacBookPro8,2) which I upgraded to 8GB RAM last October (2x 4GB Corsair DDR3 1333Mhz).

I drive about 30km twice a week between my flat and my weekend home. In this time I'm carrying my MBP in a backpack (tilted 90° resting on the superdrive slot-side).

Recently things have gone strange - after starting my MBP it will act up greeting me with the typical 3-beep sequence of

a failed RAM Integrity Check. Sometimes it will happen straight after power on, sometimes it'll take a few minutes.

The only thing to get back to working is to reseat both DIMMs and that's it - after that the error is gone and won't reappear until I do the backpack + drive

routine again.

I made sure to run various memtest runs on each of the DIMMs - with both in and also with only one DIMM in each of the RAM slots.

All tests suggested the RAM is fine so I tried to swap it for the original RAM supplied with the MBP - same 3-beep-after-driving-around issue 😟

TBH I don't really know what to think about this.. I'm afraid my logicboard could start to fail on me but then why does a simple reseat always fix it?

Any help is appreciated.

P.S.

I already did a PRAM and SMC reset (several times..)

Message was edited by: Shirkrin - fixed spelling..