2008 RAV4 (4-cyl.) Engine Shudder

We have a 2008 Toyota RAV4 (4-cyl.) With just under 34,000 miles. Some months ago, the engine began to shudder from time to time as the vehicle slowed near the end of a stop. Recently, we took it in to a nearby Toyota dealership to have the problem looked at while it was still covered by the basic warranty. They claimed they couldn't find any issues (no codes present). We had them look at it again after the issue persisted and again. Nothing. The "Check Engine" light has never come on, though this continues to happen intermittently. When it does, the engine shudders noticeably and the RPMs drop to 500-550 (even less during the more severe occurences). What could be causing this?