Why does my car lose power at random times?

I have only had my Mercedes C220 CDi SE Avantgarde 3 months (second hand) 2005 model.
I tend to drive steady and never hard, the problem is on the average when im on a motorway doing 70mph and I decide to increase speed to say for example to take over and need instant speed my car does not increase in speed regardless if i gently push down on the accelerator or fully push down.
There are no sounds out the ordinary and can see the speed is decreasing rather than increasing and then slowly builds back up at a very slow pace but the gears are changing as if you was intending to drive slow and have slow gear change.

My car has full service history and i have had it serviced only a week ago on time but problem has not gone away although rare I am concerned it may get worse .

I have tried driving in C and S mode but makes no difference.

I thought the idea behind buying a decent car was performance but this does not deliver, my corsa gave more performance than this.

PS I have had to select a ramdom model from the drop down as mine is not listed.