A friend sent me his old 486 machine from Australia, and I decided to fix it up. It came with an AM486DX 40mhz, 32mb of RAM, a Longshine LCS-6941 VLB caching controller, a SB16 Vibra, a Cirrus Logic graphics board, some random modem and network boards, and a generic controller with COM and and LPT ports (and IDE/Floppy, both disabled). For whatever reason, a CD-ROM drive won't work on the Longshine's secondary IDE port, but it did work fine on the SB16's IDE port.

I decided to upgrade it to an Intel 486DX2 66, which required me to change the FSB from 40mhz to 33mhz (via motherboard jumpers.) Since doing that, I've had problems with the CD-ROM drive. When starting up DOS, the CD-ROM loads the driver and says that the device is found, but then immediately hangs and locks up the system. If I remove the generic controller with the COM ports, the CD-ROM drive detects normally and everything runs fine (however there's no PS/2, so I need the COM port for mouse.) I tried using the IDE from the generic controller, disabling the SB16's IDE. This resulted in the exact same problem. None of this happened when I was running at 40mhz, so I'm not sure why the FSB would cause a problem. Any ideas?