Pc Freezing with Geforce GT 1030

Hi everybody,

I hope someone with more experience than me could help me to solve a problem started some weeks ago in my PC. Is going to drive me crazy and I do not understand how to get through.

Following my configuration:
CPU: AMD Ryzen 5 1600
Graphic Card: Gigabyte Geoforce GT 1030
RAM: 2 x 8giga Crucial 2400
HD: SSD Samsung 850EVO
PSU: Seasonic S12II-520 Bronze
SO: Windows 10 / Ubuntu 18.04

I would like to point out that my PC has worked without problems for one year . Now is freezing every 5-15 minutes randomly. I know this is a sneaky problem that can have many causes.
I am going to list the causes I thought about and how I excluded them after trials:

1)Virus/Rootkit: I installed last version of Malwarebytes, full scan did not report any problem

2)OS: I thought about some new update of windows 10 but I excluded cause I have the same problem on Ubuntu 18.04 installed in a different partition.

3)RAM: I was able to run Windows memory test several times without error, in some case even the test on boot freezed without any memory error. I was never able to complete memtest 86 UEFI (from USB boot) cause it always freezed at different percentage without give any error

4)Disc: I run as much tests as possible without receive any error

5)Graphics Card: excluded the previous point I focused on GC. I removed the driver with DDU and installed latest version from nvidia site, no improvement. I changed the slot for PCIE2 16x to PCIE3 4x. The PC stops freezing but goes in suspension mode randomly even If I setup power management to performance and disabled suspension, problem is the same both in Windows and Ubuntu. In despair I thought I had in the shelf a very old graphics card Sapphire Radeon HD4350. I installed it in PCIE2 16x, miracle everything fixed, I was even able to complete memtest 86 UEFI without any problem and error. I thought GT 1030 was broken, I asked RMA, I receive brand new I installed it in PCIE2, all the problem started again.
One thing I noticed, even if the GT 1030 is connected to PCIE2 x16, GPU-Z says is PCIE at x4.
Weird, it says that the old HD 4350 use PCIE x16 2.0 @ x16 2.0
Considering I have the same problem on Ubuntu I think is an hardware problem.
GT 1030 is brand new just received in substitution of the old one.
Memory passed all the test, one stick at once, when the system is with the old graphics card.
Maybe motherboard is the problem? Should I ask RMA?
Really I do not know what else to think about and I?™m asking help.

Thanks in advance for reply