“GPU has fallen off the bus”
This is a typical nvidia issue, known for more than 10 years. Depending on the XID (take a look in the nvidia doc`s) the cause is either the nvidia driver or hardware.
The most common causes are:
Defective GPU.
GPU not properly seated in the PCI(e) port.
Dirty PCI(e) contacts of the GPU.
Defective PCI(e) port on the mobo.
Dirty PCI(e) port on the mobo.
Unstable PCI(e) bus, caused by a weak, inferior or defective power supply and/or connected defective UPS. All it takes is for the PCI(e) port to receive 0,4-0,5 volts too little and you will get this error.
In this case, it is always advisable to measure the voltage at the PCI(e) port or to test the GPU in a second PCI(e) port, if available.