Can you tell us what sizes are the cakes? Sometimes if they are too close in size your dowels are too close to the edge and weakens it, especially in a tapered topsy turvy. I have had the best luck with tapered topsy turvys with at least a 3" difference, for example a 9" & 6" cake (I bake a 10", 9", and 8" for the bottom layer, and a 7", 6", 5" for the top) - I use the middle layer as the size reference. So the bottom of the top tier is 5" and I carve the hole out of the top of the bottom tier, which is a 10" so there is actually a 2.5" border around the bottom of the top tier. If you only have an 1" or so around the top tier's base I think that might be contributing to your problem.
Other things to look at - I strongly recommend using ganache for tt cakes - fantastic and holds everything together or if using bc chill it, chill it, chill it!
Best wishes, these cakes can be exasperating, I know. And, a really moist cake is also not the way to go - I add white or dark chocolate to my cake batter for topsy turvys - this makes for a firm, poundcake like texture at room temp, but because of the cocoa butter in the chocolate, it melts in your mouth. The Aussies only use very dense mud cake and ganache and they are fantastic at this style of cake (as well as many others), follow their lead.