[MartinP]

During long exposures the negative gets hot and pops in the opposite direction. If that happens, you need to let the set heat up past the popping point and then expose.

If that's the case I'd suggest fitting the heat-filter, which could be missing from the enlarger.

Also, in general a glass neg carrier is the most effective way of holding a negative flat as springy small negs, or saggy large ones, will not rest in a plane but will be curved.

Double-check for a guide roller or pin that may have come out of a slot or groove when you moved the enlarger head to the top of the column. If the problem really is vibration, and any change in the setup of the enlarger is not found, then attach the top of the column to the wall in some way - as suggested above - and use a clamp or other method to hold the baseboard steady on the table (the table legs should be on a hard floor, not carpet of course).