2001 Ford Focus

Engine Performance problem
2001 Ford Focus 4 cyl Two Wheel Drive Manual 94000 miles

Hi people, wondering if someone could possibly give me advice as to what the problem is. I own a 1.4 ford focus (94,000 miles). For about 2 months it has seen the garage twice as the maintainece light came on. Both occassions had the same symptoms. Sluggish acceleartion & movement between gears and every now & then I hear a pop. The mechanics changed the spark plugs but still hasn't fixed the problem. Someone PLEASE HELP! Every time I go above 70 it starts to shake a little and I hear a popping sound. I got it chipped 2 years ago, I am wondering if this is the cause & I need to revert back to my old filter, however they assured me it wasn't this. Any ideas?
October 21, 2008.

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