Hi Ankit Shard,
To answer your immediate question - NO, the GMAT does NOT score you differently for taking more or less time when answering a question. The types of score drops that you describe do occur though - and the causes ARE definable. Test Day is a rather specific event, which you CAN train for. When these types of score drops occur, the two likely "causes" involve either something that was unrealistic during practice or something that was surprising (or not accounted for) on Test Day. For example, there are files of questions available online that contain many of the prompts from the Official GMAC CATs. If you practice with some of that material, then take a practice CAT and see even a few questions that you ALREADY know the answers to, then that knowledge will almost certainly impact your overall performance - which will also unrealistically 'inflate' the score result that you receive from that CAT. As another example, if you were to skip past the Essay and IR sections when taking a CAT, then you end up taking a much shorter, easier practice Test that requires less effort... which also almost always leads to an inflated score result.
Points can be won and lost rather easily on Test Day, so to maximize your performance, you have to make sure that you're not preparing in an unrealistic fashion.
GMAT assassins aren't born, they're made,
Rich