I replaced the drop command with the ones you suggested (for details download fireworks_2_1.intro.zip) only to find out that the problem got worse. With these commands the code only worked approximately 1 out of 10 times in an otherwise non problematic environment.
After playing around with the code for a while I found out that the problem was probably caused by the quick deceleration of the particles. Don't ask me why, but by changing the "Rubbing" and "MoveVelocity" parameters (for details download fireworks_2_2.intro.zip) I could get the code to run properly even under demanding circumstances.
The result can be seen in
this mission.