hesitation

[wombat]

Hi to all . I have a Passt estate tdi se 1900cc 2002 year and I'm experiencing a few problems.
When i accelerate, through the gears at normal speed the car hesitates before serging then flat spotting again.
When i floor it the car goes great!
Also when i hold it at 2000-2500rpm in 3rd or 4th gear the car is serging . It's a bit hard to explain but when i go at a slow steady pace the car surges and when i press the accelerate partway down the car is slow to respond.

Please help! if you can make any sense of this.

Thanks Andy