Stopcock turns off, needs closing off then back on to regain pressure

The house stopcock under the sink appears to be closing off when there is a sudden demand in the house, toilet flush for example. I have replaced the stopcock and fitted it the same way as the original with arrow in the direction flow, which although I can see enough piping, looks like the correct direction. This has happened only in the last year or so, since we moved in ten years ago. Any suggestions?

Are you certain there isn't some other device that might be causing the problem? Re-check the direction (look for the drain c*ck on the house side), if correct then it's probably a loose jumper or washer. Dismantle to inspect and repair or replace as appropriate.

The OP says he's already replaced the stopcock. I'd try turning it round. They are designed to prevent back-syphoning.

True, but new items can be faulty - although I admit that it's unlikely for a new one to have the same problem as the old one.

There may be nothing wrong with the new stop c*ck, as it's displaying the correct symptoms for a working stop c*ck that is installed teh wrong way around. The old one may have worked for ten years because the jumper was seized up inside! :)

Tim+ has given an easy way to find out which way the water is flowing, so I'd check that.