Hello I'm new and hoping for some help,

I have a 2002 Toyota Taco 4X4 V6 5 speed manual trans. I just had the trans rebuilt and I am having problems with first gear poping out when you let out the clutch. The problem is that it does not do it all the time. The trans shop is scratching their head and I'm getting frustrated. They replaced syncros for first and second gear and both sliders. Any ideas of what would cause this problem?????

Thanks