If you're simply DOWNLOADING it's a pretty basic problem. Not to troubleshoot necessarily but in terms of the system. As in its no an overheating issue that only happens when you stress the GPU or whatever.
My primary suspects are:
a) DDR3/4 memory so run MEMTEST86 for a full pass www.memtest86.com
- create the USB stick or CD/DVD
- boot (may go into BIOS)
- touch nothing and just wait for a full pass to finish or until errors
b) Power Supply - swapping to another PSU is only way to test
- i'd guess NOT the issue since DX10 works which isn't that much power difference and as well it freezes when just downloading? It's one thing if it's random PSU failures but both random AND under load seem a bit of a stretch.
c) Software corruption... not sure why basic downloading would cause a problem that also freezes on GPU usage. So doesn't seem like a graphics card issue.
*Best way to test software corruption IMO is to have a spare SSD or HDD (60GB or larger) then simply do a clean install of Windows (don't bother with login info). Install drivers and stress test.
Have all OTHER drives and devices unhooked that are not necessary.
a) insert 8GB+ USB stick
b) download and run the Microsoft Media Creation Tool (Google) for W10
c) choose "for other PC" and "W10 64-bit" and wait to download and complete
d) shut down and unhook all drives except one you want to overwrite
e) boot to W10 Install, DELETE any existing partitions so you have just unallocated or whatever
f) Install W10, drivers etc and test
If you don't have a spare HDD or SSD try finding on cheap. Really handy. I keep a basic W10 install handy just to test things if they go wonky using an old 60GB SSD that's otherwise useless.
d) motherboard?
Eliminate everything else first.
e) Graphics card?
Possibly, though why DX11 would crash when DX10 works seems illogical to me plus the downloading issue should be unrelated to the graphics CARD then possibly somehow drivers or related software (hence the clean install method to test).
*So start with MEMTEST86.
OTHER:
**If you're really stuck and think it may be software then create the W10 USB stick, then run "setup.exe" from it inside Windows. You should see the option to keep your apps and data greyed out (so it will). Just continue and it should "upgrade" W10 to itself. That may OVERWRITE any data that's corrupted.... make sure to run MEMTEST86 no matter what and if a stick of memory is bad then you may have corrupted data on the drive.. in which case I'd sort out the DDR3/4 memory problem then try the W10 Upgrade but if you still have issues plan for a CLEAN OS INSTALL carefully.