My 2011 1.6TDCi Grand C-Max has developed an intermittent loss of power over the last couple of weeks, normally after starting from cold.

When it happens the car crawls along, and putting your foot down gives no response. There are no warning messages or lights on the dash. Then a short while later the car will suddenly rev normally, as though the turbo has just kicked in.

It's happened 3 times so far, and I've taken the car back to the dealers to sort out under warranty, but surprise, surprise, they didn't find anything wrong. (I find this is pretty much par for the course with engine problems and dealers).

There were no fault codes logged and they didn't see any obvious issues with the turbo.

The car has done over 17,000 miles, and gets plenty of long runs so the DPF shouldn't be getting clogged. I also let the turbo cool down properly after long trips. The car's not used a drop of oil, and I've only ever fueled it with premium diesel too.

It has been running a little rougher and noisier than normal over the last few months, but nothing severe enough to say it's running badly.

Any ideas what could be wrong? Thanks!