4-speed linkage on A833 locks up

[usraptor]

1970 Cuda bult 440/512, 550 HP+, original A833 4-speed and Dana 60.  Trans' was rebuilt 2400 miles ago by the previous owner.  When I accelerate hard in 1st gear and start to shift into 2nd gear, the shifter/linkage locks up and won't move causing me to quickly move over to the side of the road.  If I pull/move the shifter hard for a few seconds, the shifter/linkage starts working normally again. If I accelerate normally in 1st gear it doesn't lock up.  It also doesn't happen if I accelerate hard in 2nd, 3rd or 4th. I'm not a transmission guy but my gut is telling me it's an issue with the 1st gear shift forks inside the trans vs an external linkage issue. What do you trans experts say?  Thanks in advance.

[ec_co]

another possibility could be the shifter mechanism - is it new, rebuilt or ? Have you checked rod adjustment? There is a procedure to pin lock the mechanism, have everything in Neutral and make your adjustments - I'm on my mobile phone so it's not as easy to search and post the articles, but you'll find it. It's also in the service manual

[usraptor]

To answer your questions, the shift mechanism is original.  I took it apart and cleaned and lubricated it and it's worked fine since 2018.  Yes, I'm aware of the linkage alignment procedure and did that when I installed the trans and linkage. However, I will put it up on the lift and recheck to make sure nothing has changed.  I just find it strange that it only does it under hard acceleration in 1st gear.    

[Chryco Psycho]

The sliders in the syncro hub could be worn causing them to tip too far & lock as well

[dodj]

If it were me I would start by taking the shifter off and go over it making sure all the bushings etc are in good shape.

[Mr Cuda]

I was cleaning up some spare shifters, and upon dissasembly, the shims were burred up.
 I would agree with dodj and start with taking apart the shifter tower.
Brewers has all the parts.