Well, there was some discoloration of the 5V pins on the motherboard. I rubbed them down with a cloth and an eraser as best I could (I know I didn't get everything) and then replaced the PSU. Everything seems to be stable now, but I'll be keeping a close eye on these things in the future.
Lesson learned: if your keyboard is doing insane things and your Caps Lock light is going crazy, check your power supply!
Great to hear that it seems to be sorted! From using various "exotic" power supply setups on my Amigas, I can attest to that. I often got crazy keyboard behaviour (caps lock light flashing, stuck keys, random gibberish) on an A1200 until I checked the PSU with a scope and found it had some high frequency noise on the +5V line. Didn't affect anything else so far as I could tell, but eliminated the noise and the keyboard started working again. That one took a while to sort out!
