Most likely it's an LPG fuel-supply problem, since the Punto runs perfectly on petrol.
Check these first:
- LPG filter for blockage
- LPG pressure while accelerating
- Solenoid/filter restriction
- Regulator coolant circulation and temperature
- LPG injectors
- LPG pressure/temperature sensors
The best next step is live LPG diagnostics during a road test when the fault actually occurs. Don't replace more parts until the LPG pressure and injector operation are measured.