Do you have, or can you borrow, a code reader/scanner that shows live data? See what your short term and long term fuel trims are. If they (added together) are -10.0 or greater (more negative), you may have fuel injector issues. There is a test to check the output of each injector over a specific time period to ensure they are putting out the same amount, but the tool to do this test is pricey. You may want to have a professional do this if your fuel trims are very negative.