Sluggish and no pwer up a hill. Almost backfiring in second gear.

When in idle the car purrs like a kitten most times. This problem i have has started recntly and it was just after I filled up at a gas station. I thought it was bad gas but went throught the tank i had and have been using premium gas now. Still the same isue only it seems to shake when the transmission shifts into second. It loses power when I ascend a hill, Also, it 'pops' and makes a slight knocking noise on the right hand side of the vehicle when it does this 'almost backfiring' thing. After it accelerates after 70 it seems fine. Idling it sounds good too. No rumbling except for the odd time.
Any suggestions? I bought a new fuel filter and haven't installed it yet as I want to be sure it is this thats causing the problem. I know it's not the transmission because it shifts well when it does shift. But it fights to go up a hill or accelerate between 50 and 70 kms p/hr.
It ran perfectly before I topped up at the gas station a few weeks ago. As soon as I filled up tho and drove for a block it started to do this. Thats why it makes me think it may be the fuel filter.
Any advice you can give me on this would be great, as I use this vehicle for work and back.

Thanks :)