I know that the amount of ram your computer has is a big factor in the length
of the replays, I had a p90 with 32mb of ram and could replay almost an entire
race then I got my new puter with 16 mbs of ram and have the same problem you
are having, I just added a total of 64 mbs of ram and could replay an entire
race no prob, But I do also know that by turning some of the graphic textures
off might help until you get some more ram, I would start with the grass, and
asphalt textures, and the clouds.
Take Care
Michael Lee