Hello and welcome to Bleepingcomputer.
I have heard about and also seen your problem before.
Usually, but not always it is fixed by a simple unplugging and replugging plugs or expansion cards.
Sometimes flexing makes a connection happen that is failing do to the following: Loose plugs, corrosion build up on a connection, loose expansion cards, corrosion on the contacts on RAM/slots or slots where expansion cards are placed.
Oxidation is from a chemical reaction caused by a mixture of Oxygen, moisture and the differences between two unlike metals mated together by compression.
The object is to disturb this oxidation by making the two surfaces rub against each other, or by using a device such as a toothbrush to run the bristles through the slot, across the connectors and across the contacts to remove the white powder.
Remember those old ZIF Nintendo game consoles that required that special cleaning tool? That was Nintendo's biggest mistake was using ZIF connections between hardware and now they use DVD disks instead.
Sorry for the lecture, but I like to back up any theory I use here on Bleepingcomputer.
So I would recheck all connections through out your motherboard and computer case, not only make sure they are secure, but also replug any or all connections where two metals mate to make a connection.