AMy modeling chocolate keeps coming out greasy and limp. I've been using 7 oz chocolate 2/3 cup corn syrup.suggestions?

Your ratio of corn syrup to chocolate sounds high.

If it's coming out greasy, you may be over-mixing it. It only requires 10-12 turns of the spoon/spatula. I recommend folding it gently like egg whites. 

If the chocolate and/or sugar syrup are overheated, that can also cause modeling chocolate to separate, i.e. get oily. Make sure to go easy on the heat. 

I find that right after mixing the modeling chocolate I have to let it sit for a while to cool. You can even stick it in the fridge if you are in a hurry. 

Once it's cooled I kneed it for a bit and it smooths out. Like WickedGoodies said if you have warm hands and handle it too much it will get oily. I find I often have to set it aside to rest and cool every so often while i'm working with it.