2005 Mercedes Benz 220cdi Automatic

hi i am facing a problem with my merc c220 cdi,the problem is while i drive this happens often that the rpm doesnt cross 3,000 and the speed doesnt cross 130 kms it drops by its self.i switch the car off n start it on again n then problem its okay but when i push the accelerator down fast it moves at a good rpm n speed but it slows down again.. whats the cause of the speed going down and the rpm.when i turn it on n try again i can reach speed of 200km plus but when this problem occurs it doesnt pass 130kms.i have no fault showing on my screen i got it scanned but the scanner shows nothing..