Okay, seems to me some clarification of terms is in order. To me, "cupping" of a board refers to an inward arcing of the surface across the width of the board- that is, there's something of a trough running the length of the board. The only way I see this happening with jointing is if the blades are curved instead of straight across, so more of the center of the board width is shaved than the edges; replacing the blades should solve that. If the knives are poorly set, as Steve Neul suggested, that would create the opposite effect, i.e., more wood removed from the edges, creating a mound in the center. To me, that's not called "cupping".
This is not what I am experiencing.
If the inward arcing occurs along the length of the board, I think a more accurate term is "bowing" (shaped like *ahem* a bow). That is where issues of table geometry- coplanarity, outfeed elevation with respect to knives, etc., come into play. I suspect that your issue is with bowing.
Bowing, for me is a protruding curve, the opposite of a cup. I have decribed my problem visually in another post.
Given the finding that lighter boards aren't a problem, one thing to check is whether there is any sloppiness in the table dovetail ways; you might need to tighten or adjust the gib screws. You could also check whether the beds are coplanar while you have some heavy weights sitting on the ends of each bed- simulating a heavy board.[/QUOTE]
I will test the tables stability as you suggest. The significant weight of the boards is some kind of contributor to the problem.
Thanks for your reply!