Remember that the GMAT is not scored based solely on the NUMBER of problems you answer correctly, but rather on the level of difficulty of the problems you get right.

That being said, it sounds like you might be too focused on trying to answer EVERY question within a time frame, and this could be hurting your score. Try not working within a strict time range (as in, not 1:40 per question), because some questions simply require more time than others - a reading comp passage will most likely take way more than just 1 or 2 minutes to read and comprehend, while sentence correction problems can be tackled more quickly.

In your case, it might be more beneficial to take a few seconds after every problem to check your work (or reasoning). If you keep getting easy problems wrong in the beginning of the test, it'll probably hurt your score a lot more than if you have to guess on one or two problems at the end of the test in order to finish.

Also, keep taking practice exams - practice always helps!