Are you just using plain butercream?
If so, thicken up your normal buttercream with PS - A LOT! I make mine so that I cna actually form it into a snake for my dam with my hands. very thick and sturdy.
Then, after I dam and fill, I push everythng down pretty firmly. I usually don't let my cakes settle when I use this method, and I haven't had any problems.
To apply your dam w/thick b'cream use a tip 12 not a coupler (w/o a tip). Do not fill above the dam - keep it just below that level - you might be using too much filling. If uyou want more fill torte the cake into 3 pieces.
Also it's best if possible to fill cake and crumb coat, then let the cake settle several hours before applying finish coat of icing. This way if a buldge does appear you can smooth that icing out before finishing it.
Also, make sure that you don't apply your dam right at the edge of your cake. Put it about 1/4 of an inch inside the edge so that when it spreads a bit it just goes to the edge of the cake, not over it. I also push everything down like ThreeDGirlie and that has been working so far.
I also put the dam IN from the outer edge of the cake, 1'4 to 1'2". Don't over fill the cake........ Use a full bodied icing. I also push down on the cake after filled. I never have that problem.
Jen 
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