Lycoming IO-390 with AFP fuel injection.
Yesterday I noticed the #3 EGT started to fluctuate quickly while the others remained stable. The fluctuation range is RPM dependent: at around 1000 rpm it may fluctuate about +/- 100 degrees. At 2600 rpm it can fluctuate +/- 600 degrees but more typically 300 degrees. These fluctuations happen within a second or two and are constantly rising and falling. The engine remains smooth with no changes in RPM and no misses detected.