cyanfire

So I haven't had my Ultimaker 2 for that long, loving it so far. But it has started very consistently missing out just one layer, about 1cm into any print. Sometimes it just causes an ugly line, and the pieces loose a lot of strength as they are not as stuck together as they ought to be. It is much worse when it needs to print support structure, this is so flimsy that the nozzle movement can cause it to snap half way through the whole print, causes a plastic chaos. Anybody know why this is happening? My first thought was an extruder problem, but I find it odd that it only happens for 1 layer, always at the same height (about 1cm). I have tried releveling the bed, but that hasn't changed anything. So I am all out of ideas! Please help