What you are describing could be something overloading the power supply.
Try (with power off, of course) disconnecting the PSU from the motherboard (so it has no load), and check the voltages then.
If the readings seem more normal then, try reconnecting the motherboard, but disconnecting everything else. Keep at it one thing at a time.
It might not be this at all, but it certainly could be either simply too much draw on an undersized PSU, or one faulty device that is draining current from an otherwise perfectly fine PSU. If this is the problem, the above tests should help identify the source.
Good luck!
Mark.
Try (with power off, of course) disconnecting the PSU from the motherboard (so it has no load), and check the voltages then.
If the readings seem more normal then, try reconnecting the motherboard, but disconnecting everything else. Keep at it one thing at a time.
It might not be this at all, but it certainly could be either simply too much draw on an undersized PSU, or one faulty device that is draining current from an otherwise perfectly fine PSU. If this is the problem, the above tests should help identify the source.
Good luck!
Mark.