In my opinion - if outside of the exam atmosphere your verbal scores is good, than there is no doubt that going into the exam when you are nervous and getting v29 instead of v40 its a mistake.
Why are you so nervous at home? why do you make more mistakes under time pressure? is it only your ability to work under pressure or do you solve the questions in a wrong way that consumes too much time and make you make more mistakes?
I believe that unless you have time limits - postponing will not be a mistake. If you will do it now, before you practiced enough the exam environment and receive a lower grade than you are looking for - you will have to wait another month in order to take it again.
On the other hand - do you have the tools to "kill" the verbal problems?
It is important for you to have a complete debrief on why the v29. and how you can fix it.
bottom line - you are the only one that can answer the question you asked. You know yourself better than anyone else and you know whats your weaknesses. You should postponed if you really believe you can change something. Postponing with no real goals or path is just a waste of time.
Good luck and keep us posted.
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