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Peter, just a thought - it might be useful to get a sense of which axis the egg-shape is in. I would be inclined to compare a guided exposure of a few minutes with an unguided and then, with a third exposure with the mount not tracking at all - so you will get star trails. That ought to give you a sense of whether the elongation is caused by slop in one axis or the other at least. If the star trails are aligned the same way as the long axis of the eggs, then that's where you look for a cause; but if the egg-shape is a right-angles to the star trails, then you would look at your dec axis. If none of these helps, there is always the possibility that your camara is not flat.
Peter
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