This ever happen to anybody

I have a 2007 Volvo 670 with a cummins isx 400. so im driving up the road. I cut the lights on and my trucks starts acting funny. I'll be doing 60 mph there's a bump and the speedometer will go down to 40 mph and right back up to 60. there is no lose in speed it just feels like power is being interrupted. in a hundred miles it happened about 40 times. also my fan is cutting on even though its not overheating. it sounds like the fan is cutting on every time it feels like a drain. so im thinking that the trailer I was pulling was draining power from my tractor because when I stopped I found that I had no lights . plus the problem didn't happen until I turned on the lights.i bobtailed home about ten miles with no problem.

 

Also no vehicle messages or check ecu