Sudden bluescreens when interacting with the Steam platform

Question

Sudden bluescreens when interacting with the Steam platform

I recently made a build for a friend and things are running great, except he always gets bluescreens that indicate a hardware issue when interacting with the Steam platform. Bootscreens often occur as soon as he tries to download something (network adapter?) or after a few minutes of idle time (when having the Steam application running). Everything else, such as browsing, working (documents etc.), downloading games via the Origin (EA) platform and playing them on high graphic settings works flawlessly.

Rig

╔═════════════╦═════════════════════════════════════════════╗
║    Item     ║                 Description                 ║
╠═════════════╬═════════════════════════════════════════════╣
║ OS          ║ Microsoft Windows 8.1 Pro x64               ║
║ CPU         ║ Intel Core i5 4570 Haswell 22nm @ 3.20GHz   ║
║ Memory      ║ 8,00 GB Dual-Channel DDR @ 666MHz           ║
║ Motherboard ║ ASUSTek (Asus) B85-PLUS (Socket 1150)       ║
║ Graphics    ║ ATI AMD Radeon R9 200 Series (ASUS) 4096 MB ║
║ Storage     ║ Samsung HD322GJ (SATA)                      ║
╚═════════════╩═════════════════════════════════════════════╝

Speccy dump with more detailed information: http://pastebin.com/VcAAhdeT

Bluescreens

There are bluescreens with different errors occuring. Mostly IRQL_NOT_LESS_OR_EQUAL. Here's an example dump:

╔═════════════╦════════════════════════╗
║    Item     ║      Description       ║
╠═════════════╬════════════════════════╣
║ BCS         ║ IRQL_NOT_LESS_OR_EQUAL ║
║ BCC         ║ 0x0000000a             ║
║ Parameter 1 ║ fffffa98`0375661a      ║
║ Parameter 2 ║ 00000000`00000002      ║
║ Parameter 3 ║ 00000000`00000000      ║
║ Parameter 4 ║ fffff801`79b01e96      ║
║ Source      ║ ntoskrnl.exe           ║
║ Address     ║ 1509a0                 ║
╚═════════════╩════════════════════════╝

Solutions

I tried various things, including running several diagnostic utilities (both by Windows and vendor specific) which didn't indicate any problem, I updated every possible driver to the most recent ones (or replacing Windows included drivers with the ones directly from the HW vendor). I reinstalled the system, tried various things with hardware and/or peripherals detached/attached in numerous combinations.

What might indicate a problem is when I run the Windows Driver Verifier, I got a bluescreen after rebooting that the driver verification failed. But I don't really know how to intepret the bluescreen message and locate the driver that caused the verification to fail:

╔═══════════════════╦═══════════════════════════════════════════╗
║       Item        ║                Description                ║
╠═══════════════════╬═══════════════════════════════════════════╣
║ BCS               ║ DRIVER_VERIFIER_DETECTED_VIOLATION        ║
║ BCC               ║ 0x000000c4                                ║
║ Parameter 1       ║ 00000000`000000f6                         ║
║ Parameter 2       ║ 00000000`00000748                         ║
║ Parameter 3       ║ ffffe000`7cfd4080                         ║
║ Parameter 4       ║ fffff801`7d3e55b9                         ║
║ Caused by driver  ║ 8b035329-1544-4cf3-ae4c-d29a11cdb4cf*     ║
║ Caused by address ║ 8b035329-1544-4cf3-ae4c-d29a11cdb4cf+15b9 ║
╚═══════════════════╩═══════════════════════════════════════════╝

* looks like a GUID to me. Can I look up this somewhere? Googling resulted in nothing useful.

Link to the two crash dumps (IRQL & driver verifier): https://drive.google.com/folderview?id=0B886k8XW_l4LflV5YmRYNUNhLV9say01LUVjTi1adW9WSnpoMzQzOTMwN2MyODRjZzc0X3c&usp=sharing

Please let me now if you need any other information!

Thank you very much.

Edit: Fixed the tables. Why doesn't the StackExchange network have proper table markdown anyway :(

Edit 2: CPU-Z reports