Looking in the logs, I see this for when I was trying to get this GTX 670 working. Do they give a clue as to what is wrong? To an amateur it looks like a web driver problem.
Event: GPU Reset
Date/Time: Sun Apr 17 15:31:17 2016
Application:
Path:
OS Version: Mac OS X Version 10.11.5 (Build 15F18b)
Graphics Hardware: NVIDIA Chip Model
Signature: 20
Report Data:
NVDA(DMA): Channel exception! Exception type = 0x20 DMA Engine Error (PBDMA Error)
Channel Info: [1, 0x6, 0x0, 0x0]
Version Info: [com.nvidia.web.GeForceWeb, 10.1.1, 0x5e5f4c655b, 20639254, 346.03.10b01, 1]
Resource Manager Info:
4443564e 000000ac 404a6307 700acfb9 00000001 00000014 d3793533 46d3a4a6
4614f297 e71edccf 00088301 00000075 149a030a 820c0a00 03180913 00480150
0a010190 3a13923d 0024380a 000e0000 e0010000 14490000 00010000 10490000
00000000 04470000 00210001 c4260006 00200009 0a220000 dc480064 f43b007c
210a0001 081e13c2 18001000 8a85b28d 28002003 3803301f bd805000 01d7adf6
48028040 0000000f 4443564e
4443564e 000000ac 9f0428ce b54f874e 00000001 00000014 d3793533 46d3a4a6
4614f297 e71edccf 00088301 00000075 149a030a 820c0a00 03180913 00480150
0a010190 3a13923d 0024380a 000e0000 e0010000 14490000 00010000 10490000
00000000 04470000 00210001 c4260006 00200009 0a220000 dc480064 f43b007c
210a0001 081e13c2 18001000 8a85b28d 28002003 3803301f 80f05000 01d7afea
48028040 00000010 4443564e
4443564e 000000d0 109ff4c6 3054652d 00000001 00000014 d3793533 46d3a4a6
4614f297 e71edccf 00088301 0000009b 12f2240a 081f0a21 80102080 00182080
00280020 00380030 9c808040 00480880 05580050 030a0060 0a00149a 0913820c
01500318 01900048 923d0a01 380a3a13 00000024 0000000e 0000e001 00001449
00000001 00001049 00000000 00010447 00060021 0009c426 00000020 00640a22
007cdc48 0001f43b 13c2210a 1000081e 80a81800 200bf7bc 30082805 50003803
b7f9d0a8 804001f1 00114802 4443564e