In reply to andy28th:
You mention that you notice a big improvement in you stamina and endurance, which is great, but what about your technique? Do you think you are actually climbing with better technique than when you started out, or do you think that the improvements in grades you've made has mainly been down to your improved fitness?
I had a similar problem to you at around the same grades (along with most of the rest of the climbing population!) and found that the thing which made by far the biggest difference to my climbing was technique drills; so I'd do focussed activities at an easy grade, usually on a traverse wall that purely looked at improving my technique. Things like side-on climbing, flagging, drop-knees etc. Initially, I would spend entire sessions just doing this and once they became ingrained, I'd just use them as part of my warm up.
I won't try to explain the drills here because it would take too long to explain, but I'm sure you can find some with a bit of Googling (or, alternatively, buy Self Coached Climber, which was a massive help to me). The big one I always remember is to
always, without exception, climb with silent feet.
Different things work for different people, so there will be loads of different answers to this and, as pointed out by other posters, you need to know what your weaknesses are to really make big gains, but I found that this is what worked for me when I hit the same sort of plateau you mention.
I hope that helps; remember that challenges like this are to be enjoyed so have fun and good luck!