You mentioned a possible "manufacturing defect" could be the cause of the current drop...I was shopping for a cheap 50 watt panel a month ago, and got an earful of good advice from a local PV distributor.  One of the interesting things I learned was that  the major cause of panel [power] degradation comes from the middle sandwich layer between the glass and silicon; it's termed the encapsulent and acts as a clear cushioning layer.  Ethylene vinyl acetate I think is the stuff.  Apparently it degrades slowly under UV exposure and pretty much determines the productive lifespan of a solar panel.  

E-man