Check engine and motor vibration

I just bought a used Dodge Grand Caravan 2011 with 126 000 km

On the third day,  while on the road at about 60 km/h, the motor start to vibrate
and the check engine light was flashing in synck whith the motor vibration.

When I bring it back to the dealer, they kept it for the day.  They told me that
they where not able to reproduce the problem and that there was no code from
the computer.

Any idea on what could cause that.
Why is there no code captured from the problem?

Any suggestion appreciated

Martin