Particle emitter works intermittently

I make an intense use of particle emitters in my space combat scene, but after coding some IA scripts that uses forces to move the ships, the laser and the engine effects are showed intermittently. Furthermore, if I add more instances of enemies (increasing the instances of the classes that controls the IA and the movements), the particle effects appears even more rarely. I can't use the profiler tool, but I notice that the framerate stays over the 45fps.

I notice this problem since I begin to use Rigidbody.AddForce and Rigidbody.AddTorque in the FixedUpdate method. May this be related to the problem with the particle emitters?