No display when PC is in horizontal position

[ynvygy]

Hello,
Yesterday i opened up my PC in order to install a second HDD. (I haven't opened it in about 6 months, so it was pretty dusty ...)
And here's where the problem started, when i tried to boot it up again, there was absolutely no display, although everything seemed to work fine from outside. I started trying out the monitor, video adaptor, rams, even took out the 2nd HDD, re-plugged and 're-slotted' the video-card
while 'testing' I noticed i do get the normal display, but ONLY if the PC sits horizontally, and the video card is not screwed in the pc case slot. If i simply try to screw it or bring the PC into any other position : diagonally - 15 or 60 degrees to the ground, or vertically, i simply don't get any display on my monitor.
Unfortunately i have no 2nd video card atm, in order to do some testing. (Hopefully i'll get one in the weekend). I'm guessing it's pretty much the mobo's PCI-E slot, although i'm not totally excluding the video-card either.
Has anyone else had a similar problem before ? And what was the cause ?

 

[Keeno99]

You could try getting compressed air to blow the dust out. I would also just double check all of the connectors in the rig, even if its not related to the display.