After my engine has warmed up for around 1/2 hour, while running around 3500 rpms or higher, the engine will suddenly drop about 1000rpms for a few seconds then return back to normal. It does this frequently after the engine is warm.<br /><br />I have had mechanics come every day last week to try and fix it. They have replaced every fuel/oil filter and pump in the engine, and have tested the sensors, and replaced spark plugs, and spark plug wires. What else could it be? 