Intermittent front end Vibration 94 Volvo 850 Sed

I have a vibration that comes and goes at a regular interval. Starts at about 60+ MPH on up. Most apparent between 70 & 75 MPH. The steering wheel will visibly vibrate for about 3 seconds and become completely smooth for 1 to 2 seconds. This vibration interval repeats over and over, in Drive or Neutral. I have replaced both outer tie rod ends, driver side wheel bearing, L & R stabilizer bars, rotors, pads and calipers. Passenger side motor mount which was visibly broken, others all ok. Finally, I just replaced both axle shafts with brand new ones. Front tires are in balance and round! At lower speeds, 60 and below it is smooth as silk. Bumps sound good, no rattle noises or loose sounds. No pulling left or right. I’m STUMPED! Thanks for any help… (FYI There does not seem to be an 850 selection so I selected 940.)

Try a tire rotate, front to back to see if that changes anything.

Second vote for the tire rotation.