Hi Burkhard,
Figures 1 and 2 look a little strange - like you have some aliasing going on with your scope. If figure 3 is correct and your clock is ~120ns (8.33MHz), then you don't have enough clocks between the RD+CONVST to properly convert the input signal. Is BUSY by chance stuck high? Can you change your word length to 24-bits in the McBSP settings?