enuttage, which version do you have? NTSC or PAL?
if you have the NTSC version, the line they give you is pretty good except you can cut in sharper on the first turn and cut in closer to the rocks on the curve before the second tunnel.
i found that the keys to this test, are nailing the the second turn (the first turn after the first tunnel, the one that is blind and is sharply right), and most importantly, the angle and exit speed out of the second tunnel. watch the demonstration for these two turns, the computer does them very well.
the secret to that turn out of the second tunnel is letting go of the gas, and getting a good angle as close as u can to the left wall, so that you can hold on to the gas the entire way without going wide and hitting the wall down on the right. you'll also see that you'll need to get a great exit speed, which means that you should be doing it without screeching ur tires, which robs alot of speed. hopes this helps. also view the demonstration a couple times for this again.
if you have the PAL version and the time limit is much harder or something, then i dunno what to tell you =)
i'm sure it took me a while back when i started playing GT3 last june, but i remember this clearly cuz i gave it a try last night and it took me 3 tries, so once you get it you'll remember it very well.
i just found out the PAL version you need a 42.800 as opposed to NTSC's 43.000. that .2 sec difference is considerable. regardless, just keep at it. there's is no license test i couldn't gold so far (up to IA) with just brute perseverence and the occasional luck from trying so many times

. like most things in life, if you keep at it, things will eventually work out.