I assume you are on XP?
I use Windows 7, tried it on my notebook(Dell Precision M6500, 1.6Ghz) and on the controller (which is hardware I put together, so no brand name, ATOM 1.8Ghz D425 on Windows 7 Embedded). Both have Solid state SATA drives and onboard graphics, the notebook ATI and the controller intel, both run 100Khz without a problem, on the notebook it runs at max speed without losing a blip on the driver test with all 8 cores busy rendering at 100% CPU load. The difference between the two is that when I open and close something on the notebook it stays steady, when I do it on the controller it takes a blip which doesn't matter though since I'm using a kflop anyway.
I've been wondering if these machines having problems are using IDE or SATA as the hard drive interface and if there is a difference?
What mainboard chipsets are they using, this is most likely the problem more-so than the onboard video card, a lot of system boards with onboard video are cheap low throughput boards.
Try and get an old server or workstation and NOT a desktop, I doubt you will ever see a problem on the high IO boards, which is more important than high CPU speed.