Pulsating kitchen tap?

We recently had a deck mixer fitted at kitchen sink. When I turn the cold on low, the water comes through in unpredictable splurges accompanied by knocking sounds. It's ok when switched on full blast. Which means I can only have the tap on full or off, else it splashes everywhere. I also have to turn it on quite a lot to get any water through at all. Any ideas what is wrong?

BoiledEgg
April 2011