Have you changed brands of butter or margarine. The water content in some of diet or margarine substitutes is somewhat different. My old tried and true chocolate chip recipe uses shortening instead of butter or margarine. If I try to use anything but shortening, they turn out flat and gooey too.

Here's my recipe:

Aunt Bertha's Toll House Cookies
Cream together:
1 c shortening
1/2 c sugar
1 c brown sugar
Add:
2 eggs
1 tsp vanilla
2 c flour, add up to 1/4 c more
1 tsp baking soda
1 pkg chocolate chips
(we like to use half chocolate chips and half butterscotch chips)

Bake at 375 degrees for 10-12 minutes.