Hey Mshred,

I had the exact same condition on one of my 4 speed gearboxes...an 18 spline. It would pop out of gear upon deceleration. I removed it and inspected it and could not find anything wrong with it. So, I took it apart, and brought the 2nd gear to a very knowledgeable manual transmission guy who goes way back to the 60s with chrysler gearboxes. In about two seconds, he showed me the problem. On the Dog teeth that engage the synchro slider gear, the teeth were worn at a very very slight angle. When the gear is engaged, the ever so slight angle or ramp would allow the slider gear to slide off of the 2nd gear.

The only repair for my gear box was a new gear, which I still have not bought (still have the tranny in the same condidtion .

So, I would inspect those dog teeth for any angled wear. It should be absolutely straight, or have a slight angle that would keep the gear pushing slightly forward into 2nd gear when engaged. (When I took apart a newer Tremec tko 600 tranny, the dog teeth are machined on angles to do just that.

I hope this helps, but at the same time, hopefully your issue is not a new gear like mine.

Good luck,
Greg.