engine problem?

I have a 1998 V40 1.9TD.

When accelerating, there's a delay between accelarator pressing and car effective acceleration. After this delay the car has a good acceleration.

This is a bit annoying when changing gear. For example when changing from 2 gear to 3 gear, the car slows down a little and then starts to accelerate after 2 or 3 seconds.

Nothing more seems to be wrong, the engine not havin strange noises and have a good top speed.

What can be wrong? How can I fix it?

Thanks