2000 Daewoo Matiz chugging when accelerating

I noticed my car started chugging when i accelerated out of 2nd gear into 3rd and it had continually got worse. it now chuggs whenever i accelerate, particularly up hills or accelerating quickly to gain speed. The car doesnt seem to have as much power as before either. theres loads of problems already with the car that i aim to get fixed, such as a rattling when i pull away which started long before the chugging. it also has some problems starting, it always starts first time, but occasionally struggles and i need to give it some gas to get it going. any ideas?