1999 Kia Clarus

Engine Performance problem
1999 Kia Clarus 4 cyl Front Wheel Drive Automatic

The pick up of the car after start is very slow. Goes very slow for quite adustance and then with slow jerks picks up good speed sometimes manipulation with the 2nd gear also helps to gain speed (pick up). Where is the problem. Gears/ engine/ spark plugs?

Also.1)does mixing of different octane of fuel make any difference.
2)there's some rattling sound from the back of the car, while on a rough patch, while not so on smooth roads, could this be related (this looks like a shock problem though)

Thanks.



Please advise.
Thanks