My link went intermitant, initialy similar sort of problem but the other way around - it wouldn't work at night! It was solved by a box swap )-: sorry. I had brought my link down & plugged it straight in the back of the box, as the LED on the link wasn't on when it wasn't working. I had a spare & tried that but it wouldn't work either so I knew it was the box. Incidentaly, when the engineer called of course it was working. However whilst 'testing' he said it had gone off (I was making a brew) & said he'd swap the box. Who says they just give bad service.
Just to reiterate, a) is the led on on the link (where it plugs into the RF socket) when its not working, b) does the remote with the link work the box. If not a) then try the link directly in the box, ruling out or identifying the cable at fault. You could always try another link, but in my experience they're pretty reliable. If not b) then its likely to be the mote (does another one work the box or link) It's just if Sky come out & it's a faulty link they've not recently installed they'll probably bill you.