I hope I'm not getting annoying with these questions.

Okay, so what I am doing is using Fire to create an engine thrust flame coming out of the back of my space ship.

It looks great in still renders (plus a bulb for that nifty lens flare).

Now, when animating I want a bit of movement in the flame, but nothing drastic.  I've tried playing with the Completion setting, but even at low setting it looks like the engine is sputtering!  Too much randomness.

With no completion added the engine flame looks too static, no movement.

Any ideas?  Should I abandon using completion and maybe use another parameter?  I figure someone has to have come across this.  I can't be the only one who rushes out and does spaceship animations first thing when learning a new 3D program.  (I did the same with 3D Studio Max 12 years ago lol.)