Does anybody know the problem?

Apparantly your rotation value is biased. Or at least not evenly set for movement in different directions.

Although cos and sin are mathematically linked, if you have rotation values accross the 0 to TWO_PI range, the movement could go in all directions evenly, since both functions return in the -1 to 1 range. See the example below.

Code Example

1. float x, y, rotation;
2.  
3. void setup() {
4.   size(500, 500, P2D);
5.   background(255);
6.   frameRate(10000);
7. }
8.  
9. void draw() {
10.   translate(width/2, height/2);
11.   rotation = radians(frameCount/2);
12.   x += cos(rotation)*3;
13.   y += sin(rotation)*3;
14.   point(x,y);
15.   if (dist(x,y,0,0)>150) {
16.     x = 0;
17.     y = 0;
18.   }
19. }

1.      rotation = direction_to_point(x,y,playerx,playery);

1.   public float direction_to_point(float tempx1, float tempy1, float tempx2, float tempy2) {
2.     float xd = tempx2 - tempx1;
3.     float yd = tempy2 - tempy1;
4.     return atan2(yd, xd);
5.     }

 

Seems all right.

You could doublecheck that the comparison is done in the same coordinate system. For example in the example below, the mouse has to be corrected to be in line with the coordinate system in which the x-y is situated. Not correcting for this will result in a bias. So it's possible there are translations either for the zombies or the player that put them in a different coordinates systems (aka with a different 0,0 origin point).

Code Example

1. float x, y, rotation;
2.  
3. void setup() {
4.   size(500, 500, P2D);
5.   background(255);
6. }
7.  
8. void draw() {
9.   translate(width/2, height/2);
10.   float playerx = mouseX-width/2;
11.   float playery = mouseY-height/2;
12.   rotation = direction_to_point(x, y, playerx, playery);
13.   PVector oldPos = new PVector(x,y);
14.   x += cos(rotation)*3;
15.   y += sin(rotation)*3;
16.   point(x, y);
17.   PVector newPos = new PVector(x,y);
18.   println(oldPos.dist(newPos)); // basically 3
19. }
20.  
21. float direction_to_point(float tempx1, float tempy1, float tempx2, float tempy2) {
22.   float xd = tempx2 - tempx1;
23.   float yd = tempy2 - tempy1;
24.   return atan2(yd, xd);
25. }

Thanks, I solved it by moving the functions for updating the position to a different part of the zombie class and called the update position somewhere else.