The Grande Punto is a good well built car, its got a big car like drive and handling, the service for Fiat cars is now managed by Tata Motors so you need not worry on that front. Expect 16kmpl on the Diesel Grande Punto in overall conditions.
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.