Bicycle Mechanics - Flat spot on a front wheel.

I've just picked up an old front racing wheel with a 700 x 23 tire. When I spin it, there is an obvious downward movement at one section of the wheel. It's not a side to side wobble, just downward. When I remove the tire and tube, the wheel spins fine, at least to my eye. However, when I replace the tube and tire and pump them up, I can clearly notice a flat section about 4 inches long, almost as if the tube is sitting too deep in the rim. Both the tire and tube are fairly new and seem to be in good condition. Similarly, the rim doesn't appear to have any obvious kinks in it. Can anyone help? I've never come across this kind of problem before.

Thanks.

I have something like this on one of my wheels too, but there's a broken spoke where the wheel does this.
Have you checked the spokes for loose or broken spokes?

Any chance the tire is uneven? Try a different tire and see if the problem remains.

Thanks for your suggestions. I checked both the tire and spokes, but no obvious problems. Finally, I took out the inner tube again and this time noticed that the rim tape had ridden up slightly on one side of the rim. I'm guessing that the tire was flat and left standing for a prolonged period. As a result, the rim tape slowly crept up on one side. Anyway, I took a sharp knife to the tape and cut off the excess. Like magic, the flat spot has now disappeared. Thanks again for your suggestions.