If you are not driving more than 40-50 kms a day, the petrol should just be fine. The petrol variants of Fiat Punto are equipped with a 1.4L Fire petrol engine with a five speed manual gearbox that offers smooth riding with better fuel efficiency - it offers a mileage 12kmpl in urban areas while on the highway it gives 16kmpl . On the other hand the diesel variants of this car are equipped with Fiat's most popular 1.3L, Multijet diesel engine with variable geometry turbocharger (VGT) that enhances the engine performance along with its fuel efficiency and offers 14kmpl mileage in city and 19kmpl mileage on highway roads.
Sir, if all the engineers are saying that there is a problem in Punto than they might be right, then you should test another Punto in same variant for that problem. If you feel the same problem in the other car also than the engineers are right. But if you won't found the sane in test vehicle, then there Might be a problem with your tyre or wheel or it might be a problem in suspension system. If the problem is with tyre, then it is a regular problem. But you are saying that the problem appears when it just jumps over a bump. By this, it is sounding as a problem with suspension system.