Active asteroid fields, even if the average speed is set to zero, will still endeavor to have two to three rocks moving toward any warships in the field. When you set an average speed, you must concern yourself with the ones the field is actively throwing at friendly ships, as well as the ones that just happen to be on a collision course. Beyond that, reducing the density (either by increasing dimensions or reducing the number of asteroids, as you've already been doing) will make things a little easier. You may also consider adding additional fighters to guard the capital ship, since, as long as there are no hostiles immediately threatening the ship, they'll attack any asteroid within a couple kilometers of a ship they're supposed to protect. If it comes right down to it, you could also set up a safe area, inside the asteroid field (using the inner dimension option in the asteroid field editor) to give the pilot and the warship a breather.
My only other suggestion would be to select a warship with more hitpoints and less surface area. There is, after all, a reason the Iceni has almost as many hitpoints as a Hecate or Orion, and that is a certain battle with a Sobek in an asteroid field.
