You indicated that you put on new 8-hole, 16" rims. Were these brand-new from a reputable wheel manufacturer ?? An out-of-round wheel is sometimes next to impossible to balance throughout the entire mph band. If you suspect that this maybe the cause, dismount the tires and get a shop to due a dial-runout to check the "trueness" of your rims. Just one of those subtleties that very few think to consider since one would expect a rim to be manufactured properly.