I noticed that my scientists keep dropping like flies. Does anyone know what exactly causes this? I had one item thing being researched at 4000/10000 and another at 12000/12000, then 2 eggheads were gone after the next mission.

Sometimes they just die.

The probability of death is relative to how much you invest into scientists, but it looks like you did not pushed them so hard.

It just happens. You had bad luck.

I don't know how is it in PSX version, but on PC you can repeat some missions forever if you won't finish it.
And on PC, some of those missions has scientists.
You can start the mission, persuade scientists, then quit mission, and scientists are yours. Then you can start the mission again and again to get larger reserves.