The values you have set are the defaults for SSRS in 2005.

Those defaults are based on "available" memory which is not the same as free memory. If you have large reports that could require a lot of render time, you could be competing against SQL and against the OS.

Since you have 16GB of memory on the server and you are running SSRS on the same box, I might start max memory in SQL at 8GB. This would leave memory for the OS as well as some memory for your reports. That said, it is just a starting point.