How can I fix a slow running tap?

We moved 9months ago into a house that was empty for 18months. We replaced the bath, sink and had a new kitchen sink 7 months ago. Over the last month the water is running intermitternly to a trickle then the next time you run it it's fine! the bath,bath sink and kitchen sink are affected.They are all on one level downstairs, as it's a dormer bungalow.We have checked ball cock in loft and can't seem to find any thing wrong except quite alot of scale in the tanks.Could this be the course and if so what can be done about it?thank you in hope!