Revealing mistake: At the beginning when Michael has KITT move the truck, you can see the dolly apparatus that makes the truck's rear end swing out.
Revealing mistake: In the scene where two truckers try to smash up KITT with baseball bats, and KITT supposedly moves a bit backwards by himself, you can see someone 'helping' him near the passenger's seat.
Answer: KITT didn't choose to go off the cliff in Goliath Returns, it just happened as he tried to avoid Goliath. Just 'cus it happened, doesn't mean It's gonna cure his fear of "flying"