I've had switches in mice where the failure mode was that the button would send out multiple impulses per click, which wasn't suppressed by the mouse's deboucing. I think I replaced the main switches in the mouse twice before I retired it for a new one (it had rubber soft touch parts that were getting goey). There was no other issues - no cracked solder joints, no contamination, just wear.
Of course these mouse switches were a bit different to a keyboard, I'm just saying that this could be down to wear and tear only. Are the affected keys ones that see more use, say W, A, S and D if you're a gamer?