Well it has to be either physiological or psychological and it seems you have pretty much ruled out the physical part so that narrows it down a bit. I have had dry spells on occasion and who knows what causes them but every guy I have ever known well enough to discuss it with has them too. When it happens I think it strikes at the ego and then it becomes a performance anxiety issue and a mental block that is sometimes hard to break through. As someone suggested, I got a script for the pharamaceutical aid and when I have the issue, I take it once or twice and it seems to restore the equilibrium, the problem goes away quite quickly and I don't need the pill until the problem crops up again.