Why is my iMac randomly going to sleep after SSD upgrade?

Hi. I bought the Crucial SSD upgrade kit from iFixIt and followed this guide (27インチiMacにSSDを追加する方法 (Late 2015)) in every detail, but after changing my Fusion Drive for an SSD, my iMac has started a weird random cycle. After a certain time, in particular if I’m running many processes, the mac goes to sleep, shutting down all connections to external drives. If I press any key, it comes back only to go to sleep again. After 5 to 10 times it just turns off. Is this an issue with the thermal controller? I think it may be related with overheating.

Also, fan noise tends to be very loud now for almost anything, but it also goes down after a while.

How can I test for this and how can I fix this? Has anyone had this issue before? Thanks.