C3 Power Loss

Hi, My wife's C3 1.4HDI (92bhp) 2003/4 has had an intermittant problem since new. Sometimes when trying to accelerate away at about 1500-2000 rpm, in 2nd, 3rd or 4th gear, the engine appears to loose power and refuses to build up any revs. Re-applying pressure to the accelerator sometimes cures it but most times you need to drop a gear and force the engine above 2000rpm. This has caused a few nasty moments when pulling into traffic when the engine refuses to accelerate leaving you in the path of approaching vehicles. We have taken the car to the dealers on many occassions with this problem just to be told "we can't find anything wrong". The warranty ran out in April, and would you believe it, the problem now appears to be getting worse. Has anyone else had this problem with C3 diesels? Any help appreciated. Delboy