how can i find out what is wrong with my vailliant 824e?

Hi, can anyone help?
My boiler is 7 years old. i have an intermittent problem. When initially turning the tap on to get hot water it seems to take three attempts for the boiler to fire up before it will start heating up the water. I have tried turning the warm start on an off without any real success. It is the same problem with turning on the heating. It seems to need a couple of trial attempts before it kicks in. When it starts to work, rad's and hot water are as they should be. No air in rads and they get as hot as they should do. Two plumbers have had a look and can't diagnose problem. This appears to be a real challenge. I would be grateful for any expertise please

Heather
September 2009