RPM fluctuation at idle
 

Hey guys 06 F-250 6.0 I just bought this truck a couple weeks ago and am noticing a fluctuation of about 100 rpm while idling. It is very rhythmic and consistent. It seems to be and rpm drop instead of a surge. My lights dim and the ac/heater blower slows as well. It's about a two second drop then back up for two seconds then back down then back up....... Not sure what all has been done to the truck but it has straight pipes and appears to have an upgraded EGR cooler. 145k miles. No smoke. Plenty of power. Just want to make sure it isn't a precursor to a major issue. I plan on doing sort of a maintenance reset here soon ie all filters and fluids changed and fuel system cleaned. Any thoughts or suggestions??? Thanks!