If they are too runny, that means that they lack flour. When you put a spoonful of batter on the cookie sheet, it should look like a blob, not too thick but also not too mushy. Try using two types of flour, too - half regular flour and half cake flour for a nice consistency of batter.
Practice makes perfect. If you're not sure, try baking only one or two cookies at a time until you get the perfect blend then bake a whole cookie sheet full. Good luck!
I wrote a hub on chocolate cookies, by the way, which you may want to check out: eurocafeaulait.hubpages.com/hub/-Delicious.
Humidity can also affect the height of cookies. You can try adding a little more flour. Also, my daughter makes the best chocolate chip cookies I have ever had, and she swears by beating the butter and sugar for a long time until it is very light and fluffy.
If you want a serious answer, just skip this one. However, I suggest you feed them Sugar Cookies and fatten them up.
My mom always told me to use a 1/2 and 1/2 split of margarine and shortening for whatever amount of butter the recipe calls for and it works! Also make sure all your ingredients are at room temperature.
Since it sounds like the cookies turned out great previous to five years ago, I would wonder if you moved around that time -- namely, to an area with a significantly changed elevation. Varying elevations require varying amounts of binders and leaveners (i.e. Eggs, baking soda), and possibly changed cook times.
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