Based on the symptoms, and the correct explanation that xorbe is giving you, the following is as common as being told to reboot.
Eliminate individual hardware as the cause.
There are two ways to approach it.
(1) The first one is to disconnected all
unnecessary hardware, and see if the problem goes away. If it goes away, then reconnect one by one and when the problem reoccurs then that last reconnected hardawre is the culprit.
(2) The second is to disconnect one
unnecessary piece of hardware, and see if the problem goes away. And then another, and see if the problem goes away. Then another, ... etc. Until the problem does goes away. If the problem goes away then the last piece of unnecessary hardware disconnected was the culprit. (Hopefully a $29 DVD-RW Drive.
Or better yet a $3.99 SATA cable.
Which approach to take depends on the situation.
And cables not being properly seated is also a very common cause- with the symptoms you've described and the expansion/ contraction from temperature changes. But disconnecting unnecessary hardware can lead to exposing the culprit.
Checking the connections of the necessary hardware is a must.