Should I postpone the GMAT?

I know this is a quite popular thread here, but I'm hoping for some feedback.

My goal is at least a 700 on the test. I've finally been seeing some improvement in my tests. A long long time ago I actually got a 710 in my Kaplan CAT. Since then I had 3 straight scores of 610, 600, and 600. Finally last weekend I got a 660. On Monday (this was actually a practice test at the facility) I got a 640. My Quant has been improving, in fact I got my best score yet when I got that 660. 88th percentile (or a 49). I got 75th percentile on the 640 test. For both I got 73% correct for problem solving and data sufficiency. Difference is that for 88th percentile one I got the first 11 correct and in the 75th percentile one I got 7 of the first 10 correct only. Still both are 2 of the top 3 percentiles that I ever got in Quant (I also got 79th percentile in my 710 test).

However, my verbal has gone to hell. In my 710 test I got 98th percentile verbal. I've come to believe that that was a fluke of sorts. Since then I haven't done well at all. In the 88th percentile test for example I had a pathetic 48th percentile in verbal. Granted, I wasn't concentrated all test (I don't know why). Still my verbal percentiles haven't been very good. It was a little better in the last one, when I got 75th percentile in quant, I got 66th percentile in Math.

My question is, is it worth postponing the test? If so for how long? I mean, I know all of the Kaplan strategies for verbal. I can do some quiz banks for Kaplan also. Or I can buy the Manhattan GMAT. I don't know if the latter is worth it, since I have my Kaplan strategies. Would I be able to improve? If I should postpone it, how long is long enough but not too long? I don't want to delay too much either.