After a lot of testing with two machines, I am now at a point where I think the EHCI part of the ICH5 chipset of this machine is dying (and the USB memory sticks are still working correctly when attached to another machine).
After a lot of testing with two machines, I am now at a point where I think the EHCI part of the ICH5 chipset of this machine is dying (and the USB memory sticks are still working correctly when attached to another machine).