loaded mission crashes

hi again

i know crash issues have been reported and i've had a look at them, but still i don't know what's going on in my case...

i saved a mission (i overwrite), and when i loaded it (i was surfaced), i set to periscope depth and it crashed. if i did NOT dive but stayed on the surface it went ok (or so it seemed). it didn't crash, i played for a whike, sank a few, etc and saved. again, overwriting: not a very clever thing to do but i didn't think of it at the time.

then next time, as soon as i loaded this mission it crashed. when it says "initialising" at the beginning... well it didn't go beyond this. i have found that i can continue on the next patrol all right, so it's not too serious, only i won't be able to finish that patrol (the one which crashes) and will get no renown.

any ideas, please?? thanks all.