the game crashed every 15 minutes, then the day it worked fine, but after another day it began to crash again
lots of info given , so not much help can be given
xbox or pc for starters
have you properly restarted your PC or xbox , not just off and on
there are hundreds of threads here that might help you if you read them first
but maybe submit a ticket here and wait for official support to help you, but make sure you provide some actual info on your situation
https://support.forzamotorsport.net/