Ford Focus

[fountain]

My T-reg 1.6 Focus seems to have developed a problem. At low revs the engine is a bit juddery and lethargic, although it's not too bad and has only lost a bit of power. At high revs (above 3k) it's not as noticeable although the cars still a bit juddery under load. It idles smoothly although the exhaust makes a bit of a put-put sort of noise (not backfiring). And the exhaust gases don't smell right although i can't really describe it. There's no smoke from the exhaust though. Someone suggested it may have lost a cylinder but it just doesn't seem bad enough for that. Any ideas greatly appreciated.