The issue I see is that "not much at all" was told about the computer you put these cards into.

I run into such issues weekly (or rather "Tech" does) with 300 or less Watt power supplies on many GHz computers stuffed with everything. The all common words are "it can't be the power supply" and "it worked before".

Here's my feedback. Get in your car and go 100+MPH and my bet is that something will break that didn't before. This is about what happens when we add more to our PC and don't keep up with what may seem to many a huge power supply.

Here's a simple heat test. Most machines were built, tested with just so much in the case. You might put in a drive, that 50 Watt plus video card and it will get much warmer in the case. Try it with the cover off.