Hi Jan,
I don't have XP12 since my computer can't handle it but I have always enjoyed flying your -300 in XP11. I recently got employed by a -300 operator so I now have the pleasure of flying the real thing! Beautiful plane to fly, I'm sure you have fond memories.
To be fair to the original poster, I too had the same issue with LVL CHG in XP11. It would aggressively trim nose down to 6000fpm+ when commanded from cruise altitude - Exactly how they are describing it. I doubt it is user error since I have the same issue.
I've noticed in the real bird LVL CHG can be a little sluggish at times but nothing to this extent.
Might be worth looking into? Maybe it's a plugin conflict or something?
Just thought I would give my two cents.
Cheers