How often are you doing the interval training workouts? I've always heard that you should only do them about once a week. If you are doing them more often, you may be overtraining.
I've always seen intervals planned out as the fast part being distance based (approx 1/2 mile) with the recovery being time based (around 1 min). The times/pace you mentioned works out to about that distance with a slightly longer recovery. As far as total time to run, for someone just starting out, do 4-6 repetitions and then build up to 8-10 repetitions. From what you wrote, you've been doing them for awhile and have built up to doing roughly 8 repetitions so that's pretty good.
If you don't feel like you're getting the benefit you want and you haven't been doing them more than once a week, I'd try backing off on the number of repetitions and focus on running faster on the reps that you do. Then build the reps back up as you begin to get faster.
One thing that I found that has helped me get faster is just to run with people that are faster than me. Do you have any running groups in your area that you could meet up with once a week? Usually there's a mix of paces within those groups and you can find someone who is just a little faster than you. You might not be able to hang with them for the whole run but by pushing yourself to try to keep up, you can get faster.
Good luck and keep us posted on how your training is going.