Oh that's unfortunate. What was it that Sherlock Holmes would say..."When you have eliminated the impossible, whatever remains, however improbable, must be the truth."
If the CPU is the only thing left unchanged, then...
But before jumping to that (which is a very expensive proposition), please try this:
- Remove the internal macOS SSD.
- Remove the AMD GPU.
- Disconnect all USB devices except keyboard and mouse.
- Which keyboard and mouse are you using? At this point I can't even dismiss the keyboard and mouse!
- Disconnect all Thunderbolt devices.
- Disconnect all Windows drives.
- Connect a spare SATA SSD.
- Follow the Catalina 10.15.4 guide (including unlocking NVRAM) very carefully. One little deviation could spell doom.
- Follow the sections for Internal GPU (iGPU). Ensure iGPU is enabled and Initial Display Output is set to IGFX.
- Connect monitor to rear Thunderbolt port (if you have USB-C to DisplayPort cable) or HDMI port.
- After Catalina 10.15.4 is installed, check for stability.