2005 Defender TD5 runs and starts great until just over half a tank deisel until a quarter tank then once below a quarter tank it runs well again ?? Some black smoke when starting and accelerating. It also can cuts out a bit and sometimes stalls but only between half and quarter tank. We think it's had a new fuel pump. The fuel pressure regulator looks like it may be leaking. Will end up fixing one thing after another and not finding problem so thought I would ask if anyone experienced?