SM465 1st Gear Pop-out Problem

I am having a problem with my SM465, it keeps popping out of 1st gear unless I physically hold the shifter in 1st.

Relevant info:
engine is a Chevy 5.3 LM7
Trans is from a 1980 GM 1 ton
using the Novak adapter, to a Dana 18 TC
I rebuilt the trans with all new wear items and bushings per Novak instructions

L, 2, 3, and R all work great.

[unclequack]

That looks like a NP435 you sure it's SM 465? 1-2 slider is worn out along with the synchro rings the shift forks may be bent or just worn.

[ronrico]

Did you find an answer for this yet? I just bought a truck that has this same issue. Please let me know and save me some time. If not I'll post up here again when I get to the bottom of it

[Taj_mahaul_earth_cruiser_x]

I normally start on 2nd gear, just skip first.

[damsbb]

I got this problem with my 2014 hyndai accent . Automatic gear . Not vc gear or whatever
It happens to 1st gear and revere only, , what may happened to my car is my father pushes back and furth the car in order to ruin it or i used to leave car oils un changed for too long

I weirded out that you said its very common in third gear
In saudi arabic we use the word نتعه to describe this .. me myself searched this term in english .. took mse too long
I dont now either if its the right word to describe this sudden jerky move from the car gears or not

Im on my way to search this problem on youtube just yo see how serious its and how to fix it .. if i knew anything i update you here