I don't really think the quantities of the ingredients is in question here. Unless of course the recipe has been rewritten on a new card or typed into a computer, then I'd go back and check for accuracy. Otherwise, if that's not the case, it's worked for 30 years so obviously it's something that has changed in the ingredients themselves or an oven issue.
Yes, margarine has changed, just like ice cream has changed. They started sneaking in air, then started sneaking in non-typical ice cream ingredients or using less of the typical ice cream ingredients and more fillers and putting "frozen dessert" in tiny little letters somewhere on the packaging, etc. They also started packing in smaller amounts for some things. Our local chain supermarket recently shifted to 4 pound bags of sugar (for the same price) from 5 pound bags on the down-low. We just happened to notice last time we bought sugar.
I'd use a good unsalted butter product, check the egg sizes, and double check the sugar, as well as check the oven temperature.
Oh, also, any moves to a high altitude or vice versa? I know that makes a difference in boiling times, not sure about baking though.
Also, did you change margarine brands? I make a cookie for Christmas that my mom always made and used margarine (which I don't normally use). I made the recipe with butter and the cookies didn't come out quite right, good but not like when I was a kid. I picked up some margarine that was on sale, made them and they still weren't right. Finally, I got the specific brand my mom used to use and they finally came out like they did when I was a kid. So, once a year, I buy a package of that particular brand of margarine and make the cookies. Don't know what I'll do if they quit making it or change their formula too much.