Probably dirty or worn out keyswitches. Google info on what kind of keyswitches were used on the ///. From the form factor, and estimated date of manufacture, I'm guessing mechanical switches soldered onto a PCB. A quick test would be to pull a keycap and take a peek.

Once you determine the keyswitch type, you'll be ready to source replacements from a donor KBD. IIRC there have been IIe & /// KBDs on eBay, but I only spot that kind of thing in passing, lookin' for MacGoodies! [] ]'>