Do you hear the change? Feel the change? If neither, the cluster itself can fail in way that'll do exactly what you're seeing.
If you're sure the RPMs actually are fluctuating, it could be the MDS kicking on and off. You can get a jump of anywhere between 100-300 rpm when it switches on or off. Make sure you're using the correct oil - 5W20. If you're on the edge of a condition where MDS would engage, sometimes it'll pop on and off as the vehicle changes speed or throttle slightly; when it happens, try tapping cruise up or down a couple mph and see if that evens things out.
2006 WK Laredo 4x4 4.7L
1993 ZJ Laredo 4x4 4.0L — Dead, sold, and rolling again