iveco splutters flt out in 3rd and 4th and blows smoke?

i have a 2001 daily, changed tank recently since then it will drive perfect in 1st,2nd and 3rd until im flat out, very hard to get it to go over 45mph at the minute, it drives fine then every so often coughs and blows white white/blueish smoke out, ive tried redex and new filter one full tank later its still happening please help