ps4 transmition automatic upshifting way to early - ps4

when i modify a car to over 500bhp (roughly) the automatic transmition gets bugged and it up shifts way too early making me loose speed and some times doesnt down shift when i slow down for a corner so i will be in 6th gear doing 60mph then after 5seconds of it struggling it finally downshifts the only way i can play at the moment is in manual, it doesnt mater what car i usem i have tryed deleting the game and re installing it but no look, its the reason i sold the game a week after it came out then i purchased it again last week hoping it would of been fixed but no look, i have spoke to a few friends about this and no one else has had this problem, and help would be muchly appreciated and its on ps4.