Leadinggol

My problem is that I have not been able to configure the graphics properly in Yosemite. I can only boot & login in safe mode. Without safe mode I always get a black login screen but with an operational mouse cursor.
I have tried various combinations. With all of them I can at best manage to boot in safe mode, in which case the system is then fully stable. The display is at full resolution (1920x1080) but the graphics is noticeably slow in normal use and hopeless with video. Using the instructions in other threads on the forum, I have installed the NVidia drivers to try to fix this but to no avail. I have looked through many posts but can't find quite the same symptoms that I have.
It seems no matter what I do, if i boot without -x flag then the login screen is not visible. In some very rare cases i have managed to type in my password and then the graphics is displayed after logging in. Usually this does not work. Once i did get into the desktop this way and saw that the NVidia 'web driver' was running (i could run the NV config tool to check, in safe mode this will not run). Nonetheless, the graphics in this case were not right anyway - flickering on web pages etc., overall worse than i get in safe mode.
When I try connecting to this box remotely using VNC, this also displays a black screen in normal mode but works in safe mode. I can get in using SSH, and from there ran kextstat to show what was loaded in normal mode. The graphics related entries seem to be:
com.nvidia.NVDAStartup (10.0.1)
com.apple.iokit.IOGraphicsFamily (2.4.1)
com.apple.AppleGraphicsDeviceControl (3.7.21)
com.nvidia.web.NVDAResmanWeb (10.0.1)
com.nvidia.web.NVDAGK100HalWeb (10.0.1)
com.nvidia.web.GeForceWeb (10.0.1)
com.apple.driver.AppleGraphicsControl (3.7.21)
com.apple.driver.AppleGraphicsDevicePolicy (3.7.7)
com.nvidia.CUDA (1.1.0)
Perhaps there are other diagnostics I can run from the remote shell?
I would be happy with the setup if i could boot in normal mode but perhaps not have the full acceleration of the GPU, so long as general use of 2D apps is fine for web, video etc.
Further details:
OS:
OSX 10.10.1 (14B25) Kernel Version 14.0.0
I think this is using Clover.
I have tried boot options:
nv_drv=1 with and without
GraphicsEnabler=Yes and No
InjectNvidia true and false
InjectIntel true and false
For the combinations that boot, none work without safe mode.
Current flags:
-x -v slide=0 dart=0 kext-dev-mode=1
Hardware:
HP xw6600 Workstation with:
Dual Xeon 5240 (4-core)
24Gb DDR2 ECC DRAM
Broadcom 5755 Gigabit NIC
Realtek ALC262 Audio
(Note, xw6600 has no onboard graphics)
NVidia K600 PCIe card (Kepler GPU). One 1920x1080 monitor connected, will connect 2nd later.
Intel DC S3500 240Gb SATA SSD. Disk controller set to legacy IDE compatibility mode at present.
A SATA HDD with a backup OSX installation (in the same state as described here)
THANKS IN ADVANCE!