dual display with 90 degree rotation works only after tinkering

I just purchased a new mac mini released late July 2011. I am using it with two Dell 20" FP displays which I had used successfully with my old powermac G5 without trouble. The displays are oriented in potrait mode to fit into the hutch on my desk. I configured the system on the mini, which saw both displays easily, but when i went into potrait mode (e.g. 90 degree rotation) the strange behavior begins.

When I first start the machine, the second display shows either the background or just a dull gray screen but I cannot move windows onto the display (I am not mirroring) nor can I change its settings using system preferences. If I lower the res on my first display, the second one comes up, with some tinkering I can get it to work, but it seems only if I hold my mouth right.

After I get it working, it seems to work ok. Anyone else have similar trouble? How can I get this fixed?