At Seca, I'm betting the "corkscrew" is what's giving you the most problems. At the top of the hill get on the brakes at the second "Bosch" sign and go gently down the hill.
For the 90 degree turn before the start/finish line get on the brakes at the "4" or "3" marker. then get thru the turn slowly. Don't get full on the gas till you have the car going straignt.
At the beginning of the track the switchback "Andretti turn" is also a booger. You don't want to be going much more than 45 MPH thru this turn. If you are playing on a PSone you'll have to rapidly tap the gas to maintain speed. On the PS2 just lighten the pressure on the gas and maintain about 45-50mph thru the turn.
The 155 at Rome-night will just require getting familiar with the track. You can win the track in Arcade mode and use it for practice. You will be faster as you learn the track. Or you can keep trying the test.
The Europa at Grindenwald: Well, there are two approaches. Use the europa you win in the historic car races, tuned similarly and practice, practice, practice. You are looking for smoothness. You want to maintain speed thru the corners. I feel the "make or break" point in this test it the near 180 degree turn near the end.
If you can negotiate this turn without spinning, AND manage to maintain speed you can beat this one.
The Peugot also requires a smooth run. It's hard because the car is so dang slow, leaving little margin for error. It will require much repitition, and muttering.
I personally had the most trouble with the GT 40 one at Seattle, And it's harder on GT-3.