Felicia fuel issue!

Hi, I own a Skoda Felicia Pickup 1.9d and it has developed a fuel problem recently and was wondering if anyone could suggest what the problem could be. The problem is, when im driving along the road at low rev`s in 4th gear at 30mph, it seems to be misfiring/spluttering. It is like its away to cut out but when you accelerate it goes away but it stills seems there are flat spots when acceleration too. When changing gears aswell, I have noticed out of nowhere there is like a sudden fuel rush when i havn`t even touched the accelerator. I have also noticed when leaving the car idling, the rev`s all of a sudden increase by themselves for a short time, then it continues idling normally again. At first I tried changing the fuel filter thinking this may cure the problem to no avail. I had the timing belt changed about 4 months ago also, so im wondering if it could maybe be timing related or something to do with that, but im unsure. Any response will be much appreciated thanks Shaun.