up and down wobbly Campag Vento

I have discovered that my rear Campag Vento wheel (2000 model) has a wobble in it (not a side to
side like it was out of true, but an up a down wobble, like the rim has an egg shape). It seems
there is a "heavy point" on the wheel, if you hold the wheel and spin it, it will eventually come to
rest with the same point always down (no particular point on the rim that I can see, i.e. not at the
joint). In fact it will actually oscillate (i.e start to rotate in both directions) before coming to
rest. It is not the tyre/tube/rim tape (I have removed them) or the QR. By my eye, the wheel is in
good condition, not out of true and the spoke and spoke heads all look fine.

The effect is more dramatic when the wheel is spun faster and if you put it on the bike and lift the
back wheel and pedal hard by hand (until my computer reads 50 kmh), the bike begins to jump all over
the place. Quite unstable. These wheels are used for TT etc on flat roads, and to be at 50 kmh I'm
usually pedalling pretty hard and moving about a bit anyway, and have not noticed this while (could
be there- dunno?).

It is currently at my LBS, where the mechanic commented that he has seen a similar (but not as
dramatic) effect on other weighty profiled rims. Anyone familiar with this ? Is it in the hub ?

Thanks

RMan

(as I was typing this it occurred to me to check if the front wheel does a similar thing...duh...,
so please don't ask me if the front wheel does a similar thing).