Sounds like your save is being corrupted by some Addon dependant/created Unit of some kind...I think you should be looking at how 'Vanilla' you are running your ArmA version firstly. Certain Addons that were constructed during previous versions of ArmA may not be as 1.09b complatible now.
Though even if this is not the case, i would be saving my mission files somewhere safe & reverting back to an ORIGINAL/ Vanilla version of Arma 1.09b without the use of
any Addons but those Official & seeing if the problem still exists. At least then your goin to be able to narrow your search down abit......If the problem don't longer exist when you try to do the same save on a 'pure/vanilla' version of the program, you know it's got somthin to do with an Additional feature you've introduced somewhere along the line.
If it works fine, well then you've solved your problem
, if not, well at least in part by discovering it's cause....I'm running both a heavily modified version of ArmA as well as a Vanilla, & i rarely encounter problems using Vanilla....when a corruption of some kind occurs in the modified version, it rarely exist in the Vanilla when i cross check.
EDIT: I'm not sayin that this is the same in your case, just that this is the first place i'd look if i had the same problem you have...