You are accessing isDead like a static variable. Use Getcomponent from gameobject which holds that playerscript instance or set is isDead static, which i don't suggest as its player, and is not statically living for the whole software life time.
I explain it pretty clearly in these two topics:
http://answers.unity3d.com/questions/550578/cant-understand-getcomponent-c.html
http://answers.unity3d.com/questions/597617/how-do-i-addsubtract-variables-between-two-differe.html
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