1998 audi a6 quattro start issues

1998 audi a6 quattro takes long time to start when sitting for a while. when i get up in the morning to start my car it takes up to 10-20 tries. at first it wont start at all and then it will start for a few seconds, sputter and die. eventually it will start but shake a whole lot. if i press the gas it will shake and pop (backfire). if i let it sit for a few minutes its fine. it drives fine after the fact. if i shut it off when i get to work and start it again, it starts right up. then later (5-8 hrs) it has the trouble again. i replaced the fuel filter with no luck…