vacuum advance unit failure is a very common problem with the Panda. Good news its easy to change! Its mounted on the side of the distributor, held on with 2 screws, one of which also holds the condensor in place. Remove these, remove the vacuum pipe from the unit and also you need to take off the distributor cap as the unit has a plastic arm that clips on inside the dissy. You can buy them from Fiat, about £20 or on shop 4 parts on the net for much less... do you have the FIRE engined Panda, if so the ones on sale are right for them, if a 903 engine then you have to re-route the vacuum pipe, all very simple stuff.
BUT FIRST.... make sure that is the problem! You can easily test the unit by removing dissy cap, exposing the plastic arm mentioned, then remove vacuum pipe from carb end and suck! The plastic arm should move, thus altering the advance mechanism, if it doesnt move its broke!