Camera stops firing externals flashes after a while

My wife is running into a pesky issue. She has a Rebel T5i with a Yongnuo YN622C trigger on it. With it she is triggering two 580EX flashes, each sitting on top of a YN622C trigger as well.

This is her description of the issue:

"I was able to shot for a while - I'd estimate about 90 minutes - but every few photos or so, one would just be pitch black and then at that point (about 90 minz), they all started being black point blank, even though all of the various flashes and flash triggers had working batteries and were firing when tested. I gave it about an hour came back, and they were still coming out black."

The occasional black shot could easily be the flashes still recharging. I'll buy that. What concerns me is the pitch black photos after 90 or so minutes of shooting products in a white box.

Since 90 minutes in both triggers and flashes are manually shooting when tested, I'm assuming it might be an issue with the camera which is admittedly not really built for heavy use. Perhaps there is some circuit protection mechanism at play?

Another interesting detail:

"When the photo just comes out black, the shutter speed sounds different. Like, it happens more slowly and is also "heavier" sounding, for lack of a better word (like if you had the shutter speedy set to, say, 1/30 or slower still, not something normal for handheld like 1/200 - 1/400), if that makes sense."

Any suggestions?