Dell Optiplex GX280 wont work with case closed

Ok. Had a weird issue today. I had to work on a system that would not power on at first, so I replaced the power supply, and then did some internal cleaning. Tested all the hardware with my diagnostic tools... all was ok. I even started up into the O.S. just to check the operating system and make sure it was up to date and had an antivirus. After everything was done, I shut it down, closed the cover and turned it back on just to verify everything was ok. .....

It did not show the Dell logo and just had the BIOS version at the top left corner of the screen, and the monitor kept flickering / degaussing kind of.

I opened the case up and powered it on without any problem! Closed the case,... same thing. Opened it back up and was looking carefully in the computer for anything that would be shorting out... but it only happened when the case was closed. Bypassed the case switch that tells if the chasis has been opened or not. Took everything off the motherboard except bare bone equipment, and it did the same thing. Used a power supply tester to test the voltages on everything when the case was closed... everything tested ok. I had the same exact model to test with, so the next step was to put this motherboard in a working computer case, and vice versa...
Well After that they both worked!

Long story short, it ended up being that the power supply cable was laying on the memory chips when the case was closed. The power cables were heavy and caused strain on the motherboard which caused the video problems and the computer not to POST. Once the power cables were strapped to the side and out of the way ... everything worked.