you are fully packing the bearings with grease right? how are you tourquing it, i always tighten the nut until its just snung (15-20 ft lbs), then i smack the rotor with a hammer (on the hub of course) to seat the bearings, loosen the nut at put it on finger tight plus a little more.
also since 1 bearing went bad, you might need to put a new bearing race in the rotor. not very hard to do. smack the old one out with a hammer and punch and use the old race to hammer the new one in. the rotor needs to come off the car for this.