[ODE] simulation speed is very variable

[michael kapelko]

i have smth like test_crash (written to work with Irrlicht)
i have 20 boxes that form a "wall", i have 10 x 2 wall, wich "falls" 
from about 6 units height.
i have a 3-wheels car (3 spheres and one box)
when i start simulation, it begins slowly, then increases a bit, and 
then, when boxes collide it becomes VERY slow, then when all's done, 
speed increases again.
i have all objects in a single SimpleSpace
does anyone know how to make it work with a "constant" speed?

thanks.