I would also recommend trying a different Kernel if this occurs on all the machines of that type, With Optiplex 755’s i’ve been testing on, the machine will freeze on a screen of garbled lines with the stock kernel due to an addon ATI Radeon card the kernel did not support. Upon using a different kernel(kitchensink fixed my video but broke multicast in 2.6.x, kitchensink of 3.6 fixed both for me, as did a core of 3.6 with the correct option removed - I believe it was Direct Rendering Manager being removed fixed my video issue)
Search for a custom kernel on the forum, or download one from the fog UI, or create your own using:
[url]http://www.fogproject.org/wiki/index.php/Building_a_Custom_Kernel[/url]
as a guide, but i’d recommend downloading and compiling a different version of the kernel from kernel.org.