"Interesting" key repeat problems in X

[TruthSeeker]

I've installed Debian testing on an oldish (PII/III celeron) PC, and compiled a custom 2.6.15 kernel. X has a keyboard problem that renders it unusable: many keys will repeat themselves rapidly and unpredictably, i.e. "usserrrrnnammeeeeeeeeeeeeeee". There is no such problem on the console. It's not just gdm; I can startx from the console, open a terminal and have the same problem. Looking at xev, either many keystrokes get generated at the instant I press the key (all showing the same "time"), or it works properly.

Additionally, sometimes after messing with various things I boot up and it works fine, only to have it go crazy again on the next boot. I've googled around to no avail. I've kept everything updated, and tried dpkg-reconfigure xserver-xorg.

lspci gives:

00:00.0 Host bridge: Silicon Integrated Systems [SiS] 620 Host (rev 02)
00:00.1 IDE interface: Silicon Integrated Systems [SiS] 5513 [IDE] (rev d0)
00:01.0 ISA bridge: Silicon Integrated Systems [SiS] SiS85C503/5513 (LPC Bridge) (rev b1)
00:01.1 Class ff00: Silicon Integrated Systems [SiS] ACPI
00:01.2 USB Controller: Silicon Integrated Systems [SiS] USB 1.0 Controller (rev 11)
00:02.0 PCI bridge: Silicon Integrated Systems [SiS] Virtual PCI-to-PCI bridge (AGP)
00:09.0 Communication controller: 3Com Corp, Modem Division WinModem
00:0d.0 Ethernet controller: Digital Equipment Corporation DECchip 21142/43 (rev 41)
00:0f.0 Multimedia audio controller: Trident Microsystems 4DWave NX (rev 02)
01:00.0 VGA compatible controller: Silicon Integrated Systems [SiS] 530/620 PCI/AGP VGA Display Adapter (rev 2a)

Here's some excerpts from xorg.conf:

...
Section "Module"
    Load    "bitmap"
    Load    "dbe"
    Load    "ddc"
    Load    "extmod"
    Load    "freetype"
    Load    "int10"
    Load    "type1"
    Load    "vbe"
EndSection

Section "InputDevice"
    Identifier  "Generic Keyboard"
    Driver      "kbd"
    Option      "CoreKeyboard"
    Option      "XkbRules"  "xorg"
    Option      "XkbModel"  "pc104"
    Option      "XkbLayout" "us"
EndSection

Section "InputDevice"
    Identifier  "Configured Mouse"
    Driver      "mouse"
    Option      "CorePointer"
    Option      "Device"        "/dev/input/mice"
    Option      "Protocol"      "ImPS/2"
EndSection
...
Section "ServerLayout"
    Identifier  "Default Layout"
    Screen      "Default Screen"
    InputDevice "Generic Keyboard"
    InputDevice "Configured Mouse"
EndSection

[TruthSeeker]

Any ideas? I tried another keyboard, but it had the same problem.
Again, the keyboard works on the console. So it has to be an X problem, right?