If you're not unintentionally increasing your portions, could you be overestimating them instead? If you eat too few calories your body will go into starvation mode and it will significantly slow, or stop, your weight loss. I was accidentally eating too few calories and ended up on a 5 week plateau.
Another possibility is that your body has simply got used to what you are doing and what you are eating. No matter how much you cut your calories your body is going to learn to adjust, which is why so many people have weight loss plateaus. If this is the case you need to shake your diet up a bit. If you have you main meal in the evening, try having it at lunch instead. If you have three large meals split them into 6 smaller meals instead. Add protein and cut back a little on carbs. Anything to suprise your body into burning fat again.
Your body is probably used to your exercise levels as well, that's why it seems you have to continually add more and more exercise to get any results. Instead of increasing the amount of exercise you do could you change the type of exercise? Try boxing instead of rowing. Jogging outdoors instead of the treadmill. When exercise becomes easy you know that your body is used to it and is finding a way to do it in the most efficient way possible, which means you're burning fewer calories while doing it. Above all add some weight or resistance work. As the others have mentioned muscle burns more calories while resting than fat does, so increasing your muscle increases your resting metabolism which ends up being even better for continued weight loss than cardio.
Try a few different solutions and see what works. There are many causes for a plateau so you might have to experiment a bit to get your weight loss started again.
Here is a great article with advice to beat a plateau. It got me through my plateau of five weeks so hopefully it will be helpful to you too!http://www.sparkpeople.com/community/as ... ts.asp?q=8