Saved game problem

[srg]

I play a mission, save it then continue. I check the garages, all cars are in, ok and with no disappearances or vanishing paintjobs. So I reload again same game data, do one or two missions or do some tasks that takes quite a long time to accomplish, like look for oysters, spray tags,etc. Then I SAVE AS another file and continue. Now I encounter some disappearing cars, so I go back to the other last saved game, and found out that in there too, my cars are gone same as the more recent ones I had problem with. My question is, isn't that other saved game supposed to be ok since I checked it first and it was ok before I saved a different one? How come? Sometimes I encounter problems like this as far back as two saved game!! Where could be the problem here?