ive got a turbomax 828e
and that had simmiler probs. the heating would come on when it felt like it show numerous fault codes.
got vaillant out as it was still under warrenty
first thing enginer done was ask if some one had been out from them all ready to change a part.
on saying no he imediatley went to his van and came back with a new part, i think it was valve that switches between heating and hot water, changed that no props since.
i feel the origal parts must have been faulty and the new part modified.
on talking to someone else about this i think vaillant would change part reguadless of warrenty,
but if you phone them they will not admit to there being a design/manufactering fault but will advise you to let them send you enginer who will change part.
this did happen about two years ago though, but needless to say there would be still be some of these boilers
with fault still in mearchants.
and yours could be one of them.
sorry about spelling.