I think that declaring these temporary matrix variables in the header (and then allocating memory for them in the constructor) is problematic when using parallelism = 1 or 2, as is the case when using the BER Curve Gen./Sink blocks. When parallelism is in effect, there are multiple "sub" flowgraphs in process and therefore multiple, probably very-close-to-simultaneous, calls to encode(). I think what's happening is that these temp matrices are being overwritten in memory by the next call to encode() before the current call to encode() is done with them. I know that such repeated allocate/free calls in the encode() function is not ideal or efficient, I can't think of a better way to do this (for now) and allow for parallelism. With this change, these matrices are no longer member variables of the class, so I've removed the "d_" prefix from the variable names.