You would be amazed at how much vibration will come off a fan (especially the CPU fan) when the dust bunnies start breeding. That vibration can be enough to fubar the connection to a graphics card, ram, etc, as well as the obvious issue of some dust sneaking into one of the "slots".
So now you have a little hint of what has to be done, the thing that the vast majority of computer users never do. Clean the effer out every 3 moths or so by first stripping it down (like you just have), giving everything a GOOD blast of "sprayduster" to get the dust out, removing the CPU fan so you can clean that AND use a natural hair paintbrush to get all the gunge out of the cooling vanes of the CPU heatsink and any other heatsink you see (ALWAYS use a natural hair brush as them plastic ones may generate a bit of static leccy which, with the static present due to dust, can fry components) and then vacuum out all the dust and cack that has landed on the bottom of your computer case.
Also, make sure your power cable is connected and have a part of your skin in contact with the metal of the case. This will mean you are "earthed" (since the earth connection to the power supply will still be connected to electrical earth) and that will lower the risk of more static being generated. And don't forget to take out the power supply, open that, and clean it out too, especially the big fan on it too.
A clean computer is quieter, when you hear more noise than when it is "clean" that will mean there will be a buildup of gawdknowswhat on fans and that can/will cause issues.