Using a 2.6.36 kernel with Fedora 13 showed the problem. Using the 2.10 DDX driver with a newer kernel also showed the problem so it looks like this issue was masked with kernels <= 2.6.33. The thing to note is that Ubuntu 10.04 (with a 2.9 DDX) does not show the problem with newer kernels.
Using a 2.6.36 kernel with Fedora 13 showed the problem. Using the 2.10 DDX driver with a newer kernel also showed the problem so it looks like this issue was masked with kernels <= 2.6.33. The thing to note is that Ubuntu 10.04 (with a 2.9 DDX) does not show the problem with newer kernels.