I too have the Sig cap 231 with a Saito 150 and slimline pitts muffler. This plane has no stall tendencies at all. I can slow it way down for a nice 3-point landing with no wing drop or anything. When I assembled the plane I did have to add weight to one of the wings to get it to balance laterally. Maybe your lateral balance is off so when it starts to slow down it drops a wing.