Off/on Vibration above 70

[snaward]

I have a new to me 07 Boxster base, 58k miles. The PO put new Michelins on the car recently.

When I am driving above 70 mph, I get a quick shake, felt in the seat not the steering wheel, then back to smooth. Cycle repeats at about a 1 to 2 sec interval.

Looking around the net, sound like maybe a motor mount, since it seems to be rear end problem. Raising the spoiler earlier than 75 does not seem to make a difference. Speeding up, the cycle does not noticeably change in frequency. Top up or down does not seam to make any different either. Since it is not continuous I assume the tires are probably not the issue.

During the oil change I did not see any obvious issues with any suspension problems, but that does not mean the old rubber bushings needs replacing.

Any thought on what I need to look at.

Thanks.