strange and random shiftings of blocks that were not touched

I just saw it for the third time: blocks that are shifted and moved after I press a "save".
We have a *lot* of blocks and a lot of regions. Approximately 70 blocks of witch 30 are used in seven regions.

when I press [submit], to store setting on one block, some others (about four) blocks have changed status, weight and region. Blocks that I am sure I did not touch! The first and second time I thought it was just me, who accidentaly clicked on somthing. Yet after the third time, I am near 100% that is not that case, and that drupal hits some weird limit and starts doing random stuff.

I can see no patters, so I cannot give more detailed information at the moment.
There seems no problem with the rendered HTML.

Bèr