99 honda shadow vlx 600 has vibration at speeds over 45?

just put on new tires and had them balanced. it vibrates after i reach 45 and higher. but the vibration is not constant. about every 5 seconds it will vibrate for a couple seconds and then smooth out and then do it again. when i pull the clutch while going those speeds it still does it so i thought it might rule out engine harmonics. any ideas?