Has your ZF been serviced at all? The sliding sleeve or the gear-cone teeth are worn out, so one or the other will have to be replaced. The after market (NON ZF) 2nd and 3rd gear tops that some of the vendor sell don't have the right angle ground on the small teeth, so the sliding sleeve will not hold and will give you the same problem,thats why I ask.Please call me at RBT
and I will talk about what we do for a complete rebuild.