Intermittent water pressure upstairs

Hi,

Moved into a new house a couple months ago. I've noticed on a couple of occasions that the water pressure on the 1st floor is very weak. To the point where it just dribbles out the tap.

When left for a while, the pressure seems to return, but with a coughing and spluttering as if there was an air lock or the mains water had been switched off by the water board etc.

I've only ever noticed the problem in the evenings, and even then only once or twice.

Pressure downstairs is fine, as is hot water pressure upstairs. We have a water softener, but I don't see why that should affect the pressure; especially not only on one floor!

Any clues?

The upstairs taps will be fed from a tank in the loft,see how big(small) it is when you say it happens in the evening do you have w/machine bath shower e.c.t. going

other option is to have the tank just feeding hot water tank and put cold upstair taps on mains presure

P.S.
check the ball valve is not corroded and filling tank very slow??

Thanks -- I'll give it a once-over next time I'm up there.

It hadn't occurred to me actually - my last place was mains-fed upstairs for cold water. Can't see the advantage of a header tank to be honest?