|"I read all of the power supply voltages - OK.|
Read power supply ripple on all voltages - OK"
That's all well & good but what about amperages? There's no easy way to check for that unless you have a PSU load tester such as shown here:
All you know is that your +3.3v, +5v & +12v rails are within voltage tolerances, but are they capable of supplying 20A (for example) or just 2.0A?
If you have another power supply handy, even if it doesn't fit in your slim case, hook it up & try it.