Crankshaft pulley AGAIN!

[SaabC5]

Right, this is getting silly now. I'm about to change the cranshaft pulley on my 2.2 yet again. It was knocking when i bought it so i replaced with an aftermarket one bought from a well known motor factors, 8 months down the line this started knocking so i thought i'd replace with an OE Citroen pulley (£115!) now 14 months down the line this is knocking its tits off so it needs replacing. What i want to know is why it keeps failing on me? The serpentine belt, pulley and tensioner were all replaced with an OE kit when i bought the car and they are all functioning fine so why does the crankshaft pulley keep going?

[old'uns]

if it's any consolation, on my old 2.0 8v i replaced 3 times as many pulleys as belts, regardless of how much they were.
the shortest time one lasted was 4000m, around 4 months if i recall, replaced under warranty but still a pain.