This is a very grey area where all sorts of circuit peculiarities 
may be found.  It needs to be sorted out on the bench - with RF 
engineers, EMC specialists and others in attendance.  Skill and 
experience are required. 

There probably isn't one single cause of the behaviour and sod's 
law says that if you look at it with a 'scope:
  a) the effect of the probe will change the symptoms and
  b) what you see on the screen is different to what's going on! 

      Keith.