Hi Prasannathawait,
To start, it's important to remember that taking lots of CATs will NOT make you a better Test Taker. A CAT is really a 'measuring device' - when used correctly, it will give you a realistic score and help define your strengths and weaknesses, but it will NOT help you to fix any of those weaknesses. To raise your scores, you have to learn the necessary Tactics and put in the proper practice and repetitions. The CAT will show you whether your studies are helping you to improve or not. As such, you really shouldn't take more than 1 FULL CAT per week.
Pacing problems do not exist on their own though - they're the results of OTHER problems. If you can't hit your Score Goal under properly-timed conditions, then you have to take a good look at HOW you're approaching questions (your Tactics, note-taking, how many times you read each prompt before getting to work, etc.).
A Test Date at the end of May gives you a little over 4 weeks of potential study time. If your general 'ability level' is still in the high-500s or low-600s, then you would likely need at least another 2-3 months of consistent, guided study before you could consistently score 700+. There's no harm in taking the GMAT in a month's time, but if you're going to keep studying until you hit 700+ (or get close to it), then you could save some money - and aggravation - by pushing back this next Test Date.
GMAT assassins aren't born, they're made,
Rich