bungalowbill

Hi all, sorry for the long post - at my wits end. I've had my 2013 C4 (1.6 HDi (115bhp) VTR+ 5d) since August last year. It was a Citroen Select approved used car from a Citroen main dealer. It's now done 44k miles. This was my second C4, replacing a 2011 model. I have a problem in that, intermittently, the car is nervous, skittish and unstable at anything above 50mph. Some days I can bomb along at 70mph and the car feels assured, glued to the road, safe and stable. Just as quickly, the next day I can't get above 50mph before it feels like it wants to chuck me in the next ditch - the car seems to be floating on the road, and every bump or turn is scarey, with shimmying and lateral movement. Sometimes under breaking or acceleration, the back steps our or shimmies from side to side. The problem is, it's not all the time - as I said, I can do a day where it feels like it owns the road. The next day, I can't get the speed up because it's so unstable I'm petrified to drive it. It's not Tyres or Tyre Pressures - I've changed all four tyres for brand new premium tyres, this didn't fix it. I check the pressures regularly, and they are always spot on (33 front, 30 rear). I don't think it makes a difference whether it is full of Diesel or not (thinking it might be a weight issue) although I have felt it is generally more stable the heavier it is - but not always. The garage have taken a look at it on 4 occasions and can't find a fault - although they have only done cursory examinations of the underneath and visual inspections (without dismantling) of the suspension. There is no evidence of any damage from previous owner either. If I measure the gap between the rear tyre and the bodywork there is a marked difference between the left and right - the gap is wider on the right-hand-side, by about 5-10mm. The garage said this was fine, saying it depends on the geometry of the car when it is resting - the only way to test is to strap the car down and compress the suspension then measure it - they've done this and say the gap then matches. I took it to an independent wheel alignment centre who said the rear geometry was out so much they estimated the rear would crab by 11 metres for every KM I drove. It also cannot be adjusted. The main dealer would not accept their examination. Either way, I am not sure that is the cause as if it was that bad, I would have problems all the time - but I am only having stability issues 60% of the time. Any ideas what it could be? The dealer won't look at it again. They even suggested it was in my mind - but as its only intermittent AND I've had an identical C4 before, I know it's not a characteristic of the model. Help! Bill.