Broken Rocker Shafts

Can anyone help me, and the mechanic with this? My 1996 300tdi with 120, 000 on the clock has broken 2, yes 2!, rocker shafts in the past month within about 1000 miles of each other. It has pulled two of the studs out of the, first time bulkhead stud, second time middle stud and it looks as though it has at some point pulled the front one as well. Does anyone have any experience of this or any clues as to what might be causing it as it's getting a little expensive to keep repairing not to mention being without the bl**dy thing every few weeks.

It could have had a timing belt failure in the past.(Then the rocker shaft would have being removed to repair it)

The rocker shaft previous to you buying the car MAY have being over tightend and warn the threads slightly.

I would recomend you get ALL the bolt holes eli coiled to rejuvinate the threads and hopefully you will have no more problems.