Sounds like either you need a new plug.  Glow plugs often give out during break in and bench running.  Or a hotter plug.   It could also be a rich mixture, but I am guessing you know to peak it out and give it about 3 clicks rich, or better 300 RPM rich using a tach.   With the long run I would expect small bubbles before the fuel gets to the engine.  Its from a small amount of fuel flashing from a drop of pressure and nucleation.   As long as they are small enough that they don't cause the engine to surge or quit, then don't worry about the bubbles.</p>