Pulsating vibration at high speed

I have a 98 Corolla. It is in good mechanical condition. It has 103k miles. I recently after an oil change and tire rotation noticed a pulsating vibration at approx. 50 mph. The faster I go, the faster it gets. It is not a constant vibration but, lasts about 2 seconds with 1 second intervals. I had the tires balanced, two of the tires are new and the other two have about 20k miles on them, all in fair condition. I did put fix a flat in one of them about four months ago but experienced no problems with it afterward.

I have ruled out the wheel bearings and cv joint/axel. Any suggestions?