Like Jacki mentioned, that's because with R>42 ohms the phase is not close to 180 to have a positive feedback. The zero cancels the dominant pole. Then the system oscillates at the second dominant pole which happens to be around 13 GHz. Try to check if its second dominant pole occurs around 13GHz. Also try to change it to see if the oscillation frequency changes as well. What surprises me is whether the chain of inverters have adequate gain at 13GHz to sustain the oscillations.