Doing random things instead of a methodical approach based on standard adjustment procedures not only won't fix the problem but also introduces other issue. You have 4 choices:

1. Have a bike shop fix it, or at least diagnose the problem and give you an estimate.
2. Find a bike co-op or knowledgeable friend to assist you.
3. Google adjust rear (or front) derailleur - check the sheldonbrown and parktool results first and follow the ENTIRE procedure. In addition look up how to properly install and check shift cables.
4. Give us a much better description of the problem - when it started, if any repairs or incidents happened just prior, which front/rear combinations are problematic and under what circumstances, etc.