Hitting plateau

So I'm sure this has been discussed here before so forgive me if I'm bringing up an old topic. Anyway I'm hitting the gym 5 days a week now and I seem to have hit a plateau in my weight loss. I'm down to 240lbs. I'd like to continue to lose fat and gain muscle but nothing else seems to be coming off. I am dieting as well. I'm usually at the gym for almost two and a half hours each night but it doesn't seem like enough. I am mixing interval cardio with weight training. I am on the treadmill for almost 35 minutes and the rest of my time is devoted to lifting. While lifting I am starting with as high a weight as I can handle and do a set of between 10 and 20 reps depending g on the exercise and then after each set I drop the weight a little and add about 5 more reps to the previous set. I do three sets usually on each thing. I am also drinking whey isolate after every gym day. Am I doing something wrong g or does anyone have any suggestions as to what I can do to overcome this plateau? Any suggestions are greatly appreciated.