The cookie mystery revealed

So I tried my half batch of cookies this morning, when my spirits had recovered from last night. I was really down. You have to understand: I was somewhat famous at my old job for these very cookies with this very recipe, and I thought I had lost my touch, which is very sad. So I very carefully measured all my ingredients, kept the cookie sheets cool, and used my graduated measuring cups instead of my glass measures (even though I've checked and the difference is minimal). The half recipe called for 2 1/4 cups flour, and I added until the consistency was right, which ended up being 2 7/8 cups. Crazy. I baked two batches in my oven. And the same thing happened! Completely frustrated, I packed up the rest of it and went to my parents'. And guess what? It's TOTALLY my oven that's the problem! The ones I baked in my mom's oven are perfect and delicious, though they exhibit signs of having too much flour. Not surprising. So, I'm somewhat relieved, because I know now that it's not me and I haven't gone mad (not for that reason, anyway). But now I have to figure out exactly what's wrong with my oven and how far off the temperature is from what it's supposed to be. Grrr.