From what I've read and heard, one big cause of pickup blockage is sump joint sealant. The sump joint has no gasket, it is sealed with a proprietary sealant compound. It is easy to go overboard with this, and have bits squished out once the sump bolts are tightened. If you get drops squeezed out on the inside of the sump, they gradually cook, drop off, and get drawn to the pickup pipe mesh filter, slowly blocking it.
Two things you can take from this. One, don't overdo the sealant - a small continuous bead (2mm I think, but check it) around the joint, going round the outside of each bolt hole. Two, don't drop the sump unless you have to...