Hyundai Atos Prime Gls 2001

I bought a prime atos with 175mil km. A few days ago. I have a problem, the engine turns on well, regulates something high and has a left to leave. It is hard for him to lift rpm and pull a little on low, if I accelerate it in Vacuumo recovers. It seems that it is cold and then you want to normalize. Someone would know what can be ??? I am thinking of changing the spark plugs, putting a clean injectors in the fuel ...... will it be something in electronic injection ???