Failing tower calibration (no replies)

I'm commissioning a Durbie Prusa Mendel, using the calibration procedure in Calibration. The thin wall is okay, and the block is very clean. It had one wavy edge, but once I tightened the Y belt a little, it came out as I expected.

But the 50mm tower won't print. I'm using 3mm PLA, and I started at default filament temperatures (first layer/successive layers) 190/180, and it printed a couple of layers square, then started rounding corners. I tried 190/175, and it improved slightly, then 185/170. At that point, it prints about 8mm fairly square, then begins rounding corners. It prints the whole tower, but it's clearly distorted. Just to be crazy, I tried raising the temperature, to 195/185, and that made one corner a little better, but didn't help the rest. The tower stays more or less a tower, but the corners are rounded, not sharp.

This isn't just a problem with the tower: I tried printing an extruder pulley, and it came out looking sort of blurred in the the upper parts: the round base is okay, but the gear part is no good. This doesn't just seem to be temperature: once the z axis moves up, the x and y registration seems to go wrong.