karlos, are you using an oil called Comma?

when i had a stage 2 CTS box i had the same problem, as soon as the box got hot under heavy load it was hard to get it in and out of gears. A quick call to CTS and they told me never to use Comma oil, didnt give a reason.

i removed the box, drained the oil and replaced it with Castrol 75/90 and the gear problem went away.