Mag 20 eating pointboxes

I have an customer that have some strange problem on his engine .
The mag is set upped as MSD want it whit grounds everywhere +some extras for shore .
spark plugg gap at 0.17 and uses the mags trigger

It starts fine and runs fine and idling fine but as soon he trottel it over 4000-6000 rpm it blows the pointbox .
We put on an mag 12 i had around and we was able to run it empty whiteout any problems .

After that has he tried different boxes different 20mags new cabels pluggs etc etc .
Always ends up whit blown boxes .

What is it we miss here ? , any won have som ideas .
Frööjdh