Hello,
Is it Linea TJet or Linea 1.4 Fire Petrol Engine in which CNG is Fitted
CNG is not recommended in cars with Turbo-charger. With the fact CNG impacts in Power Output of Engine - but due to Turbo - Car has Higher Torque levels - this will impact in Car RPM and the performance of car can be severely impacted
If its 1.4 fire Petrol Engine - then suggest to visit workshop and scan in ECU for error codes, and let workshop clear those error codes
As the possibility of mis-match in Engine Performance and RPM could most likely be related to same
Hope should help !!