A specific key combination sporadically does not work on wired Apple keyboard

Get Key Codes (freeware) from the App Store.

Test both left & right shift keys, with & without other letters in combination. Especially test around w - a, q, 2, 3, 4, e, d, s. The circuit tracks tend to run in diagonals, so frequently there's a little pattern that forms if a track is damaged.

Test if the same data is received at all times, ie when your issue is happening & when it is not.

That will test for whether the issue is physically within the keyboard itself - which it sounds like it is.

Note: the option to record key ups & modifiers is harder to follow on-screen, but gives more precise information.

I don't know whether they key-caps are easy to remove & replace; many aren't on older keyboards like this one. I wouldn't recommend prying one off to clean underneath unless you're willing to risk it being permanently damaged.

Also - the massive price hike on these keyboards was fortunately quite short-lived after Apple stopped making them. You can get them for about $£€ 40 again now. They are essentially unrepairable, as they're glued together, so always having a spare can be a good idea. I find they usually last until you spill something on them or wear all the letters off ;)