mine did exactly the same and then stopped working all together, it turns out its the rpressure to the washing machine, after id faffed around ringing the water board and everyhting it turns out the filters at either end of the hose that feeds the washing machine were blocked, i took both of these out flushed the pipe through and then connected it up again leaving the filters off and it seems fine now, hope this helps!
helena, February 2011