Does this happen with any emitter you use, or a specific one only? What changes are you making?
I’m guessing that you’re either:
1) working at a relatively high frame number. If you load your whole movie and then try to add an emitter 5 minutes into the movie, you’re going to have problems. Break your movie into short clips that need the particle effects, add the effects to the short clips, then recombine in your editor.
2) making a change to the emitter that adds a HUGE number of particles. Maybe with an area emitter? Any emitter that is not a point emitter, I suggest cranking the “number” percentage down to 1 percent or so, then slowly raise it until you have the number of particles you want. Any emitter other than a point emitter (area, line, circle) will add more particles based on the area or length of the emitter.
Alan.
http://www.wondertouch.com