Wow, those driving skills, crashing through the pack and wildly swinging the camera doesn't help.
Interestingly it's perfectly stable 60fps in the last part, sunny track with pre-baked shadows and normal driving. Rain spray is the worst, but that's no surprise with ps3's difficulties with alpha to coverage. Dynamic lighting takes a smaller hit, usually mitigated by not playing in parasailing mode.
Anyway that explains why they minimized the spray in the 24 minute Nurburgring day/night/rain race. I love how they improved the headlight effects, reflective barriers can be almost blinding and red signs bathe the track in a red glow.
Btw this golden ps2 standard includes a fair bit of nostalgia too. GT4 often dropped to half resolution when you're driving close behind the pack, resulting in only every other line on the screen being drawn. It did keep up the frame rate that way though.