Random reboots Lenovo Ideacentre AIO 910 27"

Hardware: Lenovo Ideacentre AIO 910

Processor (CPU): Intel i7 6700T

Working memory (RAM): 1x IBM DDR3 8GB
Mainboard: AIO 910 27ISH
Power supply: External power supply
Case: All In One PC
Graphics card: NVIDIA GeForce GTX 940A; 2 GB onboard
HDD / SSD: IBM SSD 250GB
Monitor: 27"

Describe your problem:
Hi, I can't fix my Lenovo all in one PC. Hard drive was empty, I somehow got Windows installed after thousands of attempts. It randomly restart (or bluescreen with restart), sometimes right at the Windows logo, sometimes after Windows startup.

Windows installation only worked after I set everything to "deactivate" under Bios "CPU" tab. I could use it for a while (20min) without crashes.

I already replaced the following:

- Used motherboard from Aliexpress (Was sold as functional).
- New CPU
- New RAM (used only the new one)
- New external power supply
- New conductive paste on CPU and GPU
- cleaned everything from dust

Unfortunately I never manage to check the temperature as it keeps freezing or restarting.

I think that the reboots become more frequent with longer use. It stay in Bios without problems for a long time without crashes.

What steps have you already taken/tried to solve the problem?

- Tested RAM memory with MemTest from a boot CD, passed without error
- Run several CPU stress tests from a boot CD, runs through without problems

Could it be a broken Heatpipe?

Greetings