(FORTUNE Magazine) – It's always been lonely at the top, but tenure there is getting increasingly shaky, according to a new survey from consultants Burson-Marsteller and RHR International. CEO turnover in the FORTUNE 1,000 is at a five-year high, and Burson-Marsteller credits the corporate scandals of the last couple of years with the sharp uptick. But the new head honchos are still coming from the same places: The proportion of insider vs. outsider CEO appointments (66% vs. 34%) is virtually unchanged since the survey began in 1999. As for when companies make the big change, in 2004 the fourth quarter was by far the busiest, with 37 exchanges of the reins. Summer is slowest--only three new CEOs took over in July, compared with 16 in October. -- Kate Bonamici