Temporary notice - disabling "Steam Input" can help some users with certain controllers/wheels

Generally, this happens when the CPU is running out of processor power and tends to happen most commonly on tracks with more cars (ie. Le Mans and Spa Endurance with up to 62 opponent) - this is because the AI uses a significant amount of CPU power.

You can reduce the number of AI opponents by unselecting one or more classes (even all of them) on the Race Weekend configuration page. This is a good test to confirm this is due to CPU performance.

It is also worth trying to reduce the post effects graphics setting, special effects and reducing the number of sound effects.

You can find the minimum and recommended specs here