Have you tried setting UseSession to false and vice versa. Does this help? Are you using the chart in a web farm environment?
We would love to help, but without us being able to reproduce and debug the problem I am afraid that we're very limited in what we can offer as assistance :-(. Please provide as much detail as possible. Also, if you manage to isolate the problem in a small runnable project this will be invaluable for us in solving the problem for you.