Worst Experience - Tata ടിയാഗോ

I have been driving Tata Tiago XZA since October 2018. I was in Shimla till March 2019 and drove my car on hills, the car was fine till then as I never ran AC of the car but I shifted to plains in March 2019 and have to run my car with AC on. The car has started giving trouble. It becomes unstable in bumper to bumper traffic and often stalls. The economy of the car has dropped to 9-9.5 km/l. To my surprise, I was getting better mileage on the hills. There is a knocking sound coming from the engine during initial pickup (which goes away if the AC is off). Have informed the local authorized service station and all they have done is cleaned the air filter twice to improve average and pickup. They also insist on getting the car serviced (second paid service is due after 2000 km from now). Sometimes I feel that I have made the same mistake which my father did by buying Tata Indica, 12 years back. The service quality is still in bad shape. The technicians hardly are bothered and respond by statements like, "these 3 cylinder cars are crap". I feel like throwing the car away.