I'm really hoping someone can help me with this one!

I have a 2002 Ford Fiesta 1.3. When accelerating it will hit about 50mph and then get stuck - I can put my foot all the way down on the accelerator and it will not go any faster. It gets stuck for about a minute, then judders and suddenly shoots forward and is fine. I drive around 40 miles on the motorway each day, and it will do this when I accelerate to join, and then usually be fine. It does occassionally do it again after I've been driving for around 30 minutes.

Nobody can figure out whats wrong with it - the check engine light came on and the code said it was the lambda sensor. I replaced this and it has still not improved. I have also replaced the following parts in the last 3 months:

Spark Plugs

Ignition Leads

Coil pack

Cam Sensor

Fuel Filter

If anyone has any idea what this could be I would really appreciate the advice as I have hit a brick wall with this!