Hi Lars,
My name is Igor, it's a pleasure for me to help others and I'll try to help you.
Did you switch memory diagnostics to extended mode? If not, please do. Run Windows memory diagnostic tool and switch it to extended mode when it starts (F1 key). Note, test hang at ~21% for more than 2-3 hours means memory error too.
Check your RAM modes (voltage, frequency, timings) are in compliance with numbers in the motherboard compatibility list for your RAM model.
Try to increase RAM voltage in BIOS.