Hi Ameya
I'm struggling with something similar. HOwever, when i am just practicing or even reviewing the test, i am able to solve Quant questions really fast and accurately . So I'm guessing its something to do with nerves. Some things i already try doing - hide the timer until its the last 10-15min. In between the test if i get saturated i take half a minute (with the timer running) to close my eyes and gather my thoughts and attention.
Also, i think i see my accuracy greatly affected based on the time of the day (mornings are perfect.. evenings, after work - i tend to falter)..
Did you go through the tests you took and see if there were specific topics you were faltering on? Perhaps some targeted pratice on that might help there. Even with sample questions, try doing them in batches of 30+ to get used to concentrating for that long.
Lastly, if you are comfortable with all concepts (having done it in the past) and are already good with verbal - putting in some extra practice in the next few days might do the trick.