Why Does My New PC Keep Restarting?

I've just built a new PC, and I'm running into a frustrating issue where it randomly restarts, especially when gaming after a couple of hours, though it also happened once during normal use after less than thirty minutes. Here's what I'm working with:

- Motherboard: Gigabyte B550M K (rev. 1.1)
- RAM: KINGSTON Beast 16GB DDR4 3200MHz
- CPU: AMD Ryzen 5 5500 3.6GHz
- GPU: PowerColor RX 6650XT Fighter
- Storage: KINGSTON NV3 500GB M.2 plus SAMSUNG SSD 250GB 870 EVO
- Power Supply: Initially a Gigabyte GP-P650G 650W, but changed to RAIDMAX VORTEX BRONZE RX 700AC.

Additionally, I have three Arctic P12 MAX White fans connected via an Akasa Flexa FP5H PWM Splitter Hub. I'm not sure if I should disconnect any of them, as I'm unsure if my motherboard can handle that many fans.

Initially, I experienced a green screen followed by a restart, which I thought might be related to the GPU. I switched from HDMI to a DP cable, which resolved the green screen, but the random restarts continued.

I've updated the BIOS to the latest version without any improvement. I monitored temperatures using OpenHardwareMonitor: CPU averages around 50°C, GPU around 48°C, and SSD at about 33°C, all of which seem fine without any sign of power spikes.

I've done stress testing on the RAM with MemTest86, which completed without errors, and I've run Cinebench on both the CPU and GPU without issues. I was considering running Unigine Heaven for further GPU testing, but I'm not sure.

I swapped out my power supply thinking the old one might be the culprit, but it turns out the previous unit was actually 700W and worked fine in my old setup. So, the issue persists despite that change.

Anyone have insights or suggestions on what else I could try? Apart from the restarts, the PC functions perfectly without any lags or crashes. I'm also curious about the fan splitter setup and whether I should simplify that. Appreciate your help!