Robb, like everyone else has mentioned it sounds like a FPS issue, that particular stage has a bit detail to process, a lot of trees from memory, if you open up task manager what else do you have running in the background, when playing RBR?
If you haven't already, try shutting down non-essential applications/programs in task manager, see 'Processes' & 'Applications' tabs. This will reduce the workload on your computer and free-up ram (physical memory).
The only things i have running in the background generally when playing games is Explorer.exe & my Internet Security(firewall&AntiV). You can also safely shut down stuff like Adobe reader, Firefox etc basically anything not related to the core windows operating system (i presume your using windows?), Desktop Windows Manager can also be safely shutdown to.
If you can't be bothered shutting stuff down everytime before playing games a free program called 'Game Booster' (
http://www.iobit.com/gamebooster.html) will do this for you with one click, nice wee program and it has a small resource footprint.
Also, do you know if there are lot of programs booting up when you start your computer?, have a look at your "System Config" file (control panel/Admin tools) modify that before doing the above, then restart your computer.
If you are using a Nvidia Card you could also make adjustments in the Nvidia Control Panel/Manage 3d Setting's/Program settings/Richard Burns rally.. restore to defaults, then increase 'Maximum pre-rendered frames' to suit. Input lag in your steering wheel will occur if you increase too much.
If your still having problems, i have a program which i can give you a link to that will allow you to put your RBR installation file on the outer edges of your hard disk which will give a 10-15 frames appx boost.
Apologize in advance if I'm preaching to the choir, Good Luck
