Since you are still over 300 pounds, I would definitely up the calories to around 1500-1600 per day.
Also, I wouldn't consider what you are going through a plateau...more like a stall. It is very common for someone who is very overweight, to see very large losses in the first few weeks, and then it will slow down. It is also very common for a person losing weight (of any weight) to have a big weight drop, and then have a couple weeks where the scale doesn't move...and then another drop.
A 1-2 pound weight loss per week is considered a healthy level, but our bodies do not work like clockwork. It is very common to have a 3 pound loss one week, nothing the next, a 1 pound loss the next week, and none the next. Overall the average is a pound a week, if you average it out over the month.
Just be patient.