I have been losing weight for about 2 years now. I am 5'8", and started out at 200# straight up. I wasnt clinically obese, but my doctor was concerned. I initially wanted to lose 10 lbs. But at this point, I am happy to say that I have lost 43 lbs, I want to lose just a few more.
What you describe has been what I have been going through the entire time, but since I am not as heavy, it is in smaller chunks. So, I would lose 2 lbs in a week or two, and then go a week or two with no weight loss, then another 2 lb weight loss, then another few weeks would go by where I wouldn't lose any more weight.
Then, after several cycles of this, I found that I would "bottom out" and not lose any weight. In fact in some cases, months would go by without me losing a single pound. I finally figured out that when I stopped losing weight, it meant simply that something needed to change - either my workout (increasing cardio) or my eating habits (decreasing intake). Then, the weight would start to come off again, just like it had before.
When I initially started two years ago, I worked out by walking on a treadmill - 20 min was all I could do. Currently, I walk on the treadmill for an hour each morning (4 - 5 miles at an 8% grade, with 3 or 5 lb dumbells), I walk in afternoon, if the weather is condusive (another 2 - 3 miles on average 4 days per week), and every other day I do a resistance workout. Also, I try to keep my caloric intake to about 1700 calories per day (in reality it fluctuates between 1600 - 2200 -- and I usually have a "cheat day" on a weekly basis). So really, I can truthfully state that I walked off my 40+ lbs.
I have read that muscles have a "memory" and when they get used to performing a certain action, they become more efficient. So when this happens, you just need to "switch things up," change your routine - by lengthening it or doing something else altogether.
But, I think what you are going through is normal - and it actually means that you are making progress. Stick with it, and you will see that scale start dropping again.