Spluttering under load/rough idle

[blue._.skyes]

So heres the dilema,
When I started the car for work this morning was idleing a bit rough, noticed but didnt think too much of it at the time and made a mental note to keep an eye on things. Sarvo.driving home from work, was going up a hill and put a little extra throttle on and the car started spluttering and shaking. Like the whole car shaking violently. It continued to do it on the drive home everytime I took off from lights or applied any sort of decent amount of throttle. The best way to describe what the exhaust sounds like when it does it is a massive fart lol, only word i can think of that explains it. Then when I back off and it goes up a gear it sounds normal again.
Car is a series 2 forte, with a few basic things done to it.
Any help much appreciated!
Thanks