Mine does the same damn thing!!!!
I have never gotten the ride out of the second set of tires that I got from the factory ones!!!
I have had mine balanced twice and on the machine they look fine...at 75-80 MPH the vibration comes and goes about every couple of seconds and sometimes goes away completely for 30 seconds or so???
I think some of the guys have had them "Road Force" balanced which takes into account "hard spots" in the tires and treats them as a heavy spot...makes sense but nobody around here knows what I'm talking about!!!
Marauderjack