It would shake sometimes like I'm going over a very rough road and then would stop. But there isn't any sound, just serious shaking. 20 seconds, 10 seconds. I don't know if it's a coincidence but sometimes if I play around with the gas a little, more/less pressure, it goes away. But maybe it goes away on its own. All the tires are in good order. This only happens at high speeds, like 70-90mph, but NOT "the faster the shakier". What could this be?

 

I had a problem like this once on a long road trip. But for me it was the balancing weights on two tires were thrown off. **** poor previous tire mounting job done. Check your weights, see if they are there.

 

On my old truck it was a bad front driveshaft. Never could figure it out on my own until it started shaking horribly and wouldn't quit.