Problems with Sky box?

I've had about 4 new Sky boxes (not Sky+) in the past 2 years. It seems after the 6 month automatic cover period (insurancey-thing) is up with them, these boxes start to play up and I have to pay out �65 every 6 months to get a replacement. Why is this? It shouldn't be so. Then they keep pestering me with calls to take out their extended warranty with them, when really these boxes should last forever.