Hi
I had a very similar problem in the winter, although my engine is 1.4 petrol.
As I left work one day, the car could not get up a small hill, and only managed to do about 15 mph. After re starting the car, she managed to get home, but the revs were much lower than normal.
After a lot of parts being changed at the garage, it seemed that cleaning the fuel filter and something else that escapes me at the moment did the job. At first they changed a Lmba Probe, but this did not solve the engine revs, but did put out the engine management light. So maybe if you check the filters are clean and clear it may help?
Phil