Are your front tires a different diameter then the rear? You seldom have a tire perfectly balanced and with different diameter tires the heavy spots will match up briefly and then go out of phase as the car travels down the road giving you the couple of seconds of vibration and then smoothness you mentioned. I think this was refered to as the rollong drum effect in the text books when I was in high scool. Maybe adding some balancing beads to the tires would help. Trying a different set of tires/wheels if available would be a good idea to narrow down possible causes.