To answer your first question - Pick up some tough passages from GMATPrep. These would be found in the MGMAT forums or in documents elsewhere.
To answer your question in the subtext - your problem is too generic i.e. it doesn't mention at all what problem you are facing, and why you think RC is your Achilles heel.
Remember that RC is not as tough if you solve isolated questions. It becomes tough for a variety of reasons:
1. Unlike SC or CR where you need to remain focused for only about 90 seconds here you need to hold your breath for nearly 8minutes.
2. Either the entire passage is experimental or it is not. So in a way a passage can either be very important, or not important at all.
3. The stimulus in CR and the sentence in SC will cost you 1 question. The passage in RC will cost you as many as 3-4 questions.
4. Unlike CR, SC there are really no strategies that work very well on RC in the "heat of the moment". I mean if you start hyperventilating then pretty much you are better of making educated guesses than spending more time "solving" them.
These are just some reasons. Maybe we can help you out better if you can provide details.
Arun
