I have made round two or three layer cakes for a cousin several times.  She lives about two hours away.  No matter what I do, the layers slide.  And the icing starts to droop.  I believe it's from the vibration of the car, but not sure.  Any suggestions would be greatly appreciated.

I always have to travel about 2 hours with my cakes.  I put the cakes in a box on a large pillow on the back seat of my car.  I have read where some have a large foam rubber.  This will absorb a lot of the vibration.  If your tiers are sliding you might want to dowel through all the tiers.  But if your tiers are sliding and your buttercream is drooping, are you using a buttercream that has butter?  Are you using the same buttercream in between the layers?  I live in Florida which is very humid and hot so I need to use a good crusting buttercream with no butter.  If I used butter I am sure that my cake layers would slide and outside droop.  I hope this helps.

I didn't think about the pillow/foam.  That would probably help.  Thanks so much. Now, as far as the buttercream, I do like the flavor of butter.  Do you think butter flavoring would help it taste the same?

Sorry I did not see this before but yes it will work with butter flavoring.