fungus:
If it worked before that and doesn't work afterwards then I think we found the problem.
My problem with that statement is that it would also be correct to say : If "the complete system as a whole" worked before that and doesn't work afterwards then I think we found the problem. - we still can't say for certain that it is the power supply - although suspicion and logic says it is the most likely candidate.
The Amp sensor is not detecting any real change in the current draw by the system, so is it reasonable to assume that, once any load, no matter how large or small ( like the brand new un-used Mega ) is connected, the transformer or circuitry of the power supply could start it's gradual loss ?