vw polo 1996 1.4 8v aex?

Hi, Since replacing the battery the car will not idle correctly, it hunts between 200~1500rpm and will often just stall out, when it has warmed up the idling is better but still hunts between 500~1000rpm, I have had the coolant temp sensor tested and it is giving out the correct output (according to an autoelectician friend) There are no fault codes registered??
Before the battery change it would start and idle at 800rpm cold,hot or anywhere inbetween.