Lagging keybord; missing keystrokes

I'm on Mint 17.3 on Toshiba Satellite L500, and all of a sudden (possibly following an update), my keyboard started to act really quirky.

Some keystrokes are received with a lag, some are not, and some get doubled. This isn't tied to any particular keys.

Any ideas about what the problem might be and how to diagnose it?

This looks like a hardware/driver issue -- I get the behavior in virtual consoles too, and an exteal USB keyboard works fine. Unfortunately, using an exteal keyboard with a laptop isn't very practical.

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