NiosII continuously rebooting

Hi,

My niosII is continuously rebooting at the same code position. The Nios is running on an external SDRAM (I'm using a cyclone IV EP4CE30F484). The memory test runs OK (it looks like the memory test project size is so small that doesn't make it crash). If I add more code it reboots at a different position (so it is not related to a specific line of code - I made it crash on a printf for example). The only way I found to make it work was changing the exception vector offset (it was originally 0x000020 and I changed to 0x20020). When changing the offset I did run the memory test again on the positions where the nios was running originally and it looks fine. It fails consistently (regenerating the Qsys or recompiling made no difference, so I doubt it is a signal integrity issue). Adding more instruction cache made no difference.

An interesting thing is that it crashes at a different position when debugging.

Does anyone have any idea why it is crashing when the vector offset is set to 0x000020?

*TimeQuest timing analysis looks perfect.

Many thanks for your help