I installed second GPU (480), PC start fine, works fine, BOINC loads and starts processing tasks. After few minutes PC crashes without restart.

event logger shows

Faulting application name: boinc.exe, version: 7.6.33.0, time stamp: 0x575480dc
Faulting module name: igdrcl64.dll, version: 20.19.15.4531, time stamp: 0x57ed260c
Exception code: 0xc0000005
Fault offset: 0x000000000007a74a
Faulting process id: 0x29c8
Faulting application start time: 0x01d27bebcbfb072f
Faulting application path: C:\Program Files\BOINC\boinc.exe
Faulting module path: C:\WINDOWS\SYSTEM32\igdrcl64.dll
Report Id: 578287ef-649f-4d02-8593-c19ab798be7e
Faulting package full name:
Faulting package-relative application ID:

Faulting application name: collatz_sieve_1.21_windows_x86_64__opencl_amd_gpu.exe, version: 0.0.0.0, time stamp: 0x55eb2f35
Faulting module name: igdrcl64.dll, version: 20.19.15.4531, time stamp: 0x57ed260c
Exception code: 0xc0000005
Fault offset: 0x000000000007a74a
Faulting process id: 0x1afc
Faulting application start time: 0x01d27bebd6744694
Faulting application path: C:\ProgramData\BOINC\projects\boinc.thesonntags.com_collatz\collatz_sieve_1.21_windows_x86_64__opencl_amd_gpu.exe
Faulting module path: C:\WINDOWS\SYSTEM32\igdrcl64.dll
Report Id: 7e356426-43d6-4572-b058-84bd7c7a00cb
Faulting package full name:
Faulting package-relative application ID:

31/01/2017 21:17:55 | | Starting BOINC client version 7.6.33 for windows_x86_64
31/01/2017 21:17:56 | | OpenCL: AMD/ATI GPU 0: AMD Radeon (TM) R9 380 Series (driver version 2236.10, device version OpenCL 2.0 AMD-APP (2236.10), 4096MB, 4096MB available, 4014 GFLOPS peak)
31/01/2017 21:17:56 | | OpenCL: AMD/ATI GPU 1: Radeon (TM) RX 480 Graphics (driver version 2236.10, device version OpenCL 2.0 AMD-APP (2236.10), 8192MB, 8192MB available, 5834 GFLOPS peak)
31/01/2017 21:17:56 | | OpenCL CPU: Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz (OpenCL driver vendor: Intel(R) Corporation, driver version 5.2.0.10094, device version OpenCL 1.2 (Build 10094))
31/01/2017 21:17:56 | | Processor: 8 GenuineIntel Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz [Family 6 Model 60 Stepping 3]
31/01/2017 21:17:56 | | OS: Microsoft Windows 10: Professional x64 Edition, (10.00.14393.00)
31/01/2017 21:17:56 | | Memory: 14.71 GB physical, 32.71 GB virtual
31/01/2017 21:17:56 | | Disk: 229.31 GB total, 8.55 GB free
31/01/2017 21:17:56 | Collatz Conjecture | URL https://boinc.thesonntags.com/collatz/; Computer ID 480584; resource share 300

Faulting application name: collatz_sieve_1.21_windows_x86_64__opencl_amd_gpu.exe, version: 0.0.0.0, time stamp: 0x55eb2f35

31/01/2017 21:17:56 | | OpenCL: AMD/ATI GPU 0: AMD Radeon (TM) R9 380 Series (driver version 2236.10, device version OpenCL 2.0 AMD-APP (2236.10), 4096MB, 4096MB available, 4014 GFLOPS peak)
31/01/2017 21:17:56 | | OpenCL: AMD/ATI GPU 1: Radeon (TM) RX 480 Graphics (driver version 2236.10, device version OpenCL 2.0 AMD-APP (2236.10), 8192MB, 8192MB available, 5834 GFLOPS peak)

You are using AMD GPU-PRO driver version 16.60 ? I have a feeling that might be the problem. I would try if 16.50 worked better.

Sorry, I was wrong.... 16.60 was only for Linux and your system has Windows. So it has completely different driver.

Faulting application name: boinc.exe, version: 7.6.33.0, time stamp: 0x575480dc
Faulting module name: igdrcl64.dll, version: 20.19.15.4531, time stamp: 0x57ed260c
Exception code: 0xc0000005

The exception code means there is a problem with a driver.
The module name points to the Intel graphics accelerator.

Faulting application name: collatz_sieve_1.21_windows_x86_64__opencl_amd_gpu.exe, version: 0.0.0.0, time stamp: 0x55eb2f35
Faulting module name: igdrcl64.dll, version: 20.19.15.4531, time stamp: 0x57ed260c
Exception code: 0xc0000005

Again the exception code points to a driver problem. The module name points at the Intel graphics accelerator, the faulting application however tells you it is Collatz that crashes, not BOINC.

The i7-4770K has an Intel HD Graphics 4600 GPU built-in. Is that one disabled (from the BIOS, or in Windows device manager for instance)?
It doesn't show up in the BOINC startup messages, but this GPU is OpenCL capable.

So while BOINC is the main package on the event ID, it isn't the application that crashes your computer. That's done by either the Collatz Conjecture application, or by the Intel GPU (OpenCL) driver. Best report this on the Collatz forums.
BOINC itself doesn't use the GPU, all intensive calculations are done by the project science applications, so any trouble they find is best asked on their forums. Perhaps that they have a minimum or maximum driver version you're required to use.

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