Jumping 100 points plus in one month is difficult. I'm not saying you can't do it but it's certainly going to be an uphill battle. Are your test scores in chronological or reverse chronological order? I'm curious as to what you think drove the dip in scores if they are chronological.

The main thing you have to do is identify your weaknesses. Go back through your practice tests - what types of problems were you getting wrong? Set problems? Geometry? Sentence correction? Then you need to target those areas for further study.

As for timing, I really think doing a bunch of problems at varying difficulty levels with a 2 minute timer (MGMAT allows you to put a per question timer on their tests) would be helpful. It's not that each question should take exactly two minutes on the test, some will take one, some will need three, but you want to develop a mental "clock" -- ie, you KNOW when you're getting close to two minutes without even needing to look. Don't waste your time on problems you have no clue on - if you're still staring at a problem 1 minute in with no idea how to solve it, make a guess and move on! The most important thing to remember is that guessing is okay, that the GMAT expects you to get several questions wrong even if you're scoring high. So don't worry too much if you need to guess, just refocus on the next question and keep going.

Good luck on your GMAT!