CCE not inserting manually defined i-frames

[honfrek]

hi there,

i want to insert an i-frame at a specific point in the mpeg-file to be able to set a chapter point later when authoring. if you enter a frame#, cce (i'm using 2.66) will automatically add a second i-frame at frame# 0 with the number format 0000.
my frame# is 724, so i tried 724 and 0724. i chose 12 frames/GOP with a sequence header at each single GOP. Thus, my GOP structure is
/IBBPBBPBBPBB/.
i also tried the 'restrict auto i-frame insertion' option which is known to slightly decrease image quality. none of the above settings worked. opened in virtualdub, there's a regular i-frame at frame# 722 (regular meaning it's at the beginning of a new GOP).
Trying the same GOP-structure in tmpegenc it worked, but in virtualdub there's a regular i-frame at frame# 720 and the desired one at 724.
use tmpegenc you'll suggest, but i'd rather use cce+avisynth, since it's faster and gives better quality results. i figured it could have something to do with the quantization matrices, but that's just a guess.

Anyone got ideas/same problems/solutions to this (other than using tmpegenc) ?

[honfrek]

me again,

plz forget about the quatization matrices! it's got nothing to do with that.
it seems to me that i have to move the preview slider up to the frame i want to insert the i-frame at and then click on set. just typing in the frame# and clicking set adds frame 0000 and the typed-in frame# to it, cause the slider is at that position. normally, that shouldn't be a problem, since there's an i-frame at the beginning anyway.