What are your tire temps? Are you doing events in cold temps? I find I have to tiptoe around until my tires are well into the green.
Are you using any aids?
And, AI strength is vastly inconsistent for Group to Group, and track to track. Your experience against them in Formula Rookie or other events has basically zero to do with them in anything else. Yes, practice and work on tunes until you don't spin (practice races in Custom Event will start you on hot tires, so you can quickly test driving style and braking and tuning setups without those first few tiptoe laps), but once you have the basics dialed in, it's unlikely that your AI setting will remain the same from the Formula Rookie Events.
EDIT... treed! Fernandito's comments about tires will help as well. Forgot about the Sports in the cold.