I think that your CPU is a bit slow for GP3.
GP3 relies mainly on the CPU.
I have found that graphics cards really aren't all that relevant with this game.
Folks with 600mhz CPUs used to complain about how slow GP3 ran at times.
With my old Athlon 900 I would have the game run a bit slowly through the first corner or two of a race and wet weather was often a problem.
With my Athlon 3000xp, GP3 runs great in all weather at all times.