I would remove the mainboard, lie it down on a flat non conductive surface and check it all again.
Most of the time when things like this happen it's because of a faulty ground connection(shorting against the case or bent pins touching on the back of the mainboard) That's the next thing to look for, when squeezing the case into a tower, it's very possible to have bent some of the pins on the back of the mainboard and they now touch each other. Have a look.
Anyway, if it works outside of the tower, you know you have a mounting problem. Check all screw mounts, make sure you have propper contact with the ground plane of the mainboard (around screw mounts ) and contact to the metal of the case. Plastic clips do not work.
On the other hand, not all holes in the mainboard could be screw mounts! I have never towered a 1200, plenty of 4000. If they are not screw mounts, you will need to use plastic! Metal may short it.
Just chack and make sure whatever it is making contact with the metal case is ground plane of 1200 motherboard.
If you're not sure what I am talking about in regards to looking for ground planes etc, it may be in your best interests to get someone who know a little about circutry. It will save you from destroying your 1200.
Hope I have helped a little here.
