blinking particles

Hi I created a code that with mouse movement i can change the velocity of particles but i have a problem, I cant understand why my particles sometime blink. the particles once a few second disappear for a second and then reappear . the particles continue with their path because of that i don't think they died.

I need ideas what could happen.

BP

EDIT: hmmm i thought i posted the code . here it is

 using UnityEngine;
 
 public class SunPhotonsExperiments : MonoBehaviour 
 {
     public Vector3 NorthPole = new Vector3(-10,5f,0);//The Location the photons should move
     public Vector3 Speed = new Vector3 (-4f,1f,0f);// The speed at which the photons start to move
     public float AccelDampen = 0.04f; //The value we multiply the speed change if it far away
     public float CloseToPoleDampen = 0.4f;////The value we multiply the speed change if it close
 
     ParticleSystem.Particle[] Photons2;//Our Particles
     int MAX_NUMBER_OF_PARTICLES = 500; //For the for loop and set.
 
     float StartTime;//for future uses
 
     float Mindistance = 0.2f;// The distance from which we consider the object arrived
     float MaxForce = 5;//Max allowed acceleration
     bool StartMovingToPole = false;//Should the particles start with the speed
     float MaxAllowedSpeed = 10f; // lowest speed which allowed for particles
 
     bool SpeedChanged = false;
 
     void Start() 
     {
         StartTime = Time.time;
         Photons2 = new ParticleSystem.Particle[MAX_NUMBER_OF_PARTICLES];
     }
 
 
     void FixedUpdate()
     {
         GameObject.Find("Sun").GetComponent<ParticleSystem>().GetParticles(Photons2);//Inits the array with current particles
         //ParticleSystem ps=   GameObject.Find("Sun").GetComponent<ParticleSystem>(); //For Debugging. Delete It
         int i;// for For loop
         if (GameObject.Find("Main Camera").GetComponent<MouseMovement>().IsMouseDraged) //
         {
             StartMovingToPole = true;//StartMoving To the Pole
             if(!SpeedChanged )
             { 
                 Speed = GameObject.Find("Main Camera").GetComponent<MouseMovement>().MouseDragSpeed*0.08f;
                 SpeedChanged = true;
             }
         }
         if (StartMovingToPole)
         {
             for ( i = 0; i < MAX_NUMBER_OF_PARTICLES; i++)
             {
                 Vector3 Force = NorthPole - Photons2[i].position;//also distance
                 if (Force.magnitude > 20)// if it is very far away (like at the beggining of the particle life)
                 {
                     Photons2[i].color = new Color(10,8,0);//yellow collor for fun.
                     Photons2[i].velocity = Speed;//make the photons velocity equal to the predetermined speed
 
                     if (i == MAX_NUMBER_OF_PARTICLES-1) //initialized
                         SpeedChanged = false;
 
                 }
                 else
                 {
                     float DampenValue;//dummy for storing the dampen value
                     if (Force.magnitude>10) //medium distance. low dampening
                         DampenValue = AccelDampen;
                     else 
                         DampenValue = CloseToPoleDampen;//close distance.high dampen
 
                     if (Force.magnitude < Mindistance) //if the photon close to the location
                     {
                         Photons2[i].velocity = new Vector3 (0f,0f,0f);//
                         Photons2[i].color = new Color(0,0,0,0) ;//make them invisible
                     }
                     if (Force.magnitude > MaxForce) //if the force(accel) is too high lower it to max predetermined value
                     {
                         Force.Normalize();
                         Force*=MaxForce;
                     }
                     this.Photons2[i].velocity = this.Photons2[i].velocity+DampenValue * Force;//calculate new velocity
                     if( this.Photons2[i].velocity.magnitude > MaxAllowedSpeed )
                         this.Photons2[i].velocity = this.Photons2[i].velocity.normalized * MaxAllowedSpeed;//if after all this the speed is too high. lower it
                 }
             }
             
             particleSystem.SetParticles(Photons2,MAX_NUMBER_OF_PARTICLES);    
         }
     }
 }