Thank you Paul for the help. I will remember your note here about the roll paper, as I have 2 44" and 2 24" inch rolls I will be printing from in the larger sizes.
I think I may have solved the problem though. In carefully watching how the 9900 loads, I think I figured out what is happening. Glossy FB Al has a noticeable concave paper curl because of its baryta coating -- most papers are flat or curve convexly. The Harman is mostly flat, but the edges are turned up slightly, and this seems to be what is causing the problems.
When you load a sheet into the 9900, it takes it in the rollers, advances it down, brings it back up, and then down again into position. With Harman, if the rollers took it (the curve interfered with this too I think), it would be pulled down as normal, but when it was to be pushed back up, it hit the edge of the paper feeding slot. So it would catch on the inside of the machine, the machine would think there was a jam, it would clamp the rollers down on the paper and then stop the loading process. This would also explain why roll paper users have not had a problem -- there is no rear edge of the paper to catch on the printer feeding slot, since the paper runs all the way from the roll to the edge.
I have found that if I carefully take some of that curve out of the paper by bending it in the opposite direction, it seems to behave much better. I have only tried this a few times, so I may just be getting lucky, but it seems like this might be the issue -- with my printer at least...