Built R154 jumping out of 4th

[Ale]

Hi guys, my tuner pieced together a R154 which isn't going into 4th unless I shift firm. If it does go in, it doesn't for long with it popping out under power, cruise or coast.
At times selecting 1st gear can be a bit hard when pulling from lights.
The build is based on a box that I bought not running with smashed 3/4 shift fork.
Bearing, seals and syncros where all replaced, fitted with Marlin billet shift forks plus Cro Mo thrust washer upgrade.

Box has remote shifter ext housing with R151 shifter housing to get the shortest shifter location as possible since it's in a small car.
Its bolted to the back of a turbo rotary using a Dellow bell housing with a Tilton concentric clutch slave. Pilot bearing is a bronze bushing.

Done 1000km with the box like this, we've pulled it back down and can't see what we did wrong.

Any ideas or things to try?

[RUGER]

  to 

i was gonna say worn synchros.

RUGER

[Prismo]

Make sure the rubber shifter boot is not binding also. If it is too far forward  it will shove it out of gear. I doubt that is the problem since 2nd is not popping out of gear.

[Ale]

Got all new syncros during build