Hello,
Well, I managed to get some air conditioning in my office today, and that rectified the issue—at least for the moment! So perhaps when the humidity is too high, the ink can't dry fast enough and the paper swells on the ink side, bulging up and intercepting the print head.
Paper path restrictions, poor third party paper support, and expensive, frequently(?) replaced print heads make the Canon's tough. I am hoping that these are addressed some in the 5100/6100 versions, and that the calibration function is a winner.
I also wonder when Epson will make its move with its new high-res print head.
Many thanks, Michael Morrison