Help With Bizarre Abrupt Shutdown/Reboot Cycles Triggered by CPU Loads?

[TRTechRev]

Hello. Hoping to find some insight on this issue as Im fairly tech savvy and couldn’t find this issue scouring google. So I purchased a bulk lot of lightly used Prodesk 600 G4 (mini tower not SFF or micro) desktops from my university for low unit price. My intention was modifying them into mini budget gaming desktops as Ive done with Dell Optiplex models of similar generation (8-9th Intel, Q370). They came with i5-8500, 1x 8GB 2666 RAM, 256GB SATA, board model “Baritone Rev A”. So my hardware changes include swapping the i5 with an i7 8700 (on the official list). Swapping in another HP 8GB 2666 stick in the 4th slot (1st in 2nd) for 2 channel operation. I removed the HDD/CDR bracket, installed NVME/WiFi cards with extra motherboard standoffs, cut 2 holes in the metal front panel and drilled mounting holes to install 2X Thermalright ARGB fans. O tested multiple graphics cards yet the issue persists.

My goal for the best performance in this form factor was to split off 1x SATA from lower board to SATA cable (3x SATA) and 1x SATA from the top board to SATA (presumably for CDR drive) to an 8pin PCIE adapter and have tested this configuration with an AMD RX 6600 (non XT) and an RTX 3060. You may think its the power delivery but the issue occurs also with an RX 550 or RX 6400 (no pwr besides slot). 

so what happens is I always test CPU performance and heatsink quality by hitting the CPU with a 10min Cinebench R23 Run and monitoring temps/clocks w/HWINFO64. The system will boot up fine, load into Windows, and then I’ll go to Cinebench and click run test, and either immediately or a few minutes in, the system without warning hard shuts off, screen goes black and fans off, it then reboots itself and is either stable for a few minutes or crashes multiple times in the same way when attempting to load the desktop, this sometimes happens 4-5 times and then will behave somewhat normally in all other apps, itll run q Unigine Heaven test hitting the GPU hard, and stay on, but when loading the CPU heavily it appears to randomly trigger shutdown events. Its immensely frustrating.

ive ruled out the CPU itself, I tested with another unit with the stock i5-8600, same behavior, i noticed the bios was old so saw a newer bios that said “addressed boot errors with hp secure boot” and was hopeful that could be the issue but after updating bios (luckily no power off when trying that) the same thing still happens. Ive tried going into the bios and disabling all the OEM security nonsense features (no offense HP employees but i really despise OEM motherboards, at least your UI looks a few years newer than Dell’s but theyre equally garbo, can we set a fan curve please?) and im at a total loss for what is triggering this. Its not only during a heavy CPU load either as sometimes ill just log in and itll be idling on the desktop and do the same thing and suddenly shutdown, windows power plan set to high perf with sleep supposed to wait for 30min but this happens way faster, tried balanced with no behavior change. Its incredibly annoying and i have no idea how to fix it. Hopefully somebody knows why HP desktops like to bootloop themselves for no obvious reason? Thank you.