One other issue I had was the MTRRs. On my Dell optiplex 755 there was an overlap of one of the regions. I had to make a script to change the MTRR regions so I had a "write-combining" area.
What do you have for MTRRs and lspic -v for the video card? EG: # cat /proc/mtrr reg00: base=0x000000000 ( 0MB), size= 2048MB, count=1: write-back reg01: base=0x07d600000 ( 2006MB), size= 2MB, count=1: uncachable reg02: base=0x07d800000 ( 2008MB), size= 8MB, count=1: uncachable reg03: base=0x07e000000 ( 2016MB), size= 32MB, count=1: uncachable reg04: base=0x07d500000 ( 2005MB), size= 1MB, count=1: uncachable reg05: base=0x080000000 ( 2048MB), size= 2048MB, count=1: uncachable reg06: base=0x0d0000000 ( 3328MB), size= 256MB, count=1: write-combining lspci -v ... 00:02.0 VGA compatible controller: Intel Corporation 82Q35 Express Integrated Graphics Controller (rev 02) (prog-if 00 [VGA controller]) Subsystem: Dell OptiPlex 755 Flags: bus master, fast devsel, latency 0, IRQ 26 Memory at fea00000 (32-bit, non-prefetchable) [size=512K] I/O ports at ec90 [size=8] Memory at d0000000 (32-bit, prefetchable) [size=256M] Memory at feb00000 (32-bit, non-prefetchable) [size=1M] Expansion ROM at <unassigned> [disabled] Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit- Capabilities: [d0] Power Management version 2 Kernel driver in use: i915 ... The important bits are the "write-combining" region in mtrr and the "prefetchable" line from lspci.