Drive shaft bolts sheering off, twice after repalcement

[Greta]

Hi Folks
I m hoping someone may be able to advise. I have a T Reg Porsche Boxster 986 and my driveshaft has fallen out twice, but luckily not at any speed as it has happened while initially accelerating. I have had the bolts and brackets changed twice and this time I have only become aware of them sheering off again (2 next to each other) because I have had an oil leak so spotted while searching for its origin. Anyway I cant think what is causing the bolts to loosen and then sheer off. Does anyone have any ideas?

Thank you
Greta