But I don't get why it'll be fine for a week and then constantly crash for a day.
How have your ambient (room) temps been the last week? As Summer approaches (at least in the northern hemisphere) room temps increase too and that will directly affect internal temps.
As noted several times, you generally want front to back flow through the case. So fans in front draw air in, fans in back exhaust hot air out.
I would try with side panel fans disconnected (not spinning) as they can, without wind tunnels directing the flow) cause too much turbulence and disrupt that front to back flow. You end up with the heated air just tumbling almost in place. Or go extreme the other way and remove the side panel completely and blast a desk fan in there and see how your temps are.
In any case, you need to be constantly monitoring your temps. I use and recommend CoreTemp
for newer Intel and AMD CPUs. Speedfan is very popular but it does have problem properly matching sensors to labels.
Damn...I see GZ has all ready suggested this.
As did I, but it is worth reiterating.
Also, for the record, I always want to make sure I am providing good, clean power to my systems - especially when troubleshooting intermittent problems. So if me, I might also swap in a spare, but known good PSU and see how it goes.