With these computers now over 10 years old, ruling out dirt and oxidation is always a good place to start.
I'd suggest removing both cards and giving both the card contacts and slot a good cleaning. Denatured alcohol on a clean, lint free cloth is a good place to start for the card contacts. A pencil eraser is good if you need something a little more aggressive. The slots are harder to clean, but denatured alcohol on a lint free cloth followed by compressed air can do a decent enough job.
That's the easiest thing to rule out, and if that doesn't help we can look for other problems.
By the way, from the read of your post, it looks like you are plugging your keyboard and mouse into the expansion cards-I have a personal rule against plugging keyboards and mice into USB cards. Mine always get plugged into either the USB ports on the computer, or into the ports on an ADC display. These devices don't in any way benefit from the greater speed of USB 2.0(why I'm assuming you have USB cards), and there are plenty of downsides to using them in expansion cards. The biggest one I can think of is that they may not work until the appropriate extensions are loaded by the OS, which leaves you up a creek if you need to do anything with Open Firmware or some other startup command. I'd promptly move them to the built-in ports if this is indeed the case.
(this post typed from a Quicksilver G4 that has a USB 2.0 card installed, but has the keyboard plugged into the ADC cinema display and the mouse plugged into the keyboard)
