If I am reading you corectly the cause of your trouble is in the fact that your bike is running rich, witch will make it stumble at low engine speeds.
The spooge from you exhaust can come from rich jetting or to much oil in the fule mix. This is just some info since you didn't say what the spects of your bike are, ie: jet sizes, mix ratio and mods you have done.
If this dosen't help you can check out some of the links from the JUST KDX web page, they have alot of good info.