06 X3 shaking at speed

[ac22]

I have had my 2006 CPO for a few months now and all has been great...

But, in the last few days it has been giving me some big problems and even the dealer cant figure it out.

About 4 hours into the 5 hour drive from Toronto to Montreal, the car started shaking..It would shake between 100 and 120 kph and only when I was on the throttle or going up hill. As soon as i let off even a bit it would stop. This happened a few times but went away. Then on the way home it started again. About 2 hours into the drive and this time got even worse. Again between 100-120 and only when i was on the gas or going up a hill.

So I took it to the dealer and they drove it and checked it and all seams fine.
I know its not the wheels (balancing, ice stuck in there, bearings etc) as the shaking would only happen when on the gas. At the same speed with no gas on it would not shake. I guess i can also conclude that the car has to be hot as it occurs after i am driving for a while.

Has anyone experienced anything similar? what could it be? Drive shaft or drive shaft bearings? transmission? Any help is appreciated. It is now getting frustrating as the dealer has zero issues when they have it and drive it for a bit but it only happens after a while and they cant have someone drive it for three hours .lol.