The following line:
 _io_ports[MC6812_SCIDRL + m_Port]= m_TXBuffer[m_TxHead++];

Is generating a bad increment of m_TxHead.  The increment of the above line is being compiled as:
  LDX  _Frame (1292)
  INC  1A, X

when it should read
  LDX  _Frame (1292)
  LDX  0, X
  LDX  1A, X

In short, it is losing a layer of indirection, and updating offset the stack frame instead of offset my "this" pointer. It worked when my "m_Port" was 8 bit, but failed when I changed it to 16 bit (if that is any help). 
I am using the latest 3.1 compiler (is also on 3.01 compiler). My flags are:
CXXFLAGS +=-g -c -Os -mshort -fno-exceptions -fno-rtti -m68hc12 $(INCLUDEFLAGS)