[Crex]

Made a guess on your setup. If your small wheel sits on the end of a yoke, check where the bearings rest in the yoke. A lot of the yokes are made of aluminum and can wear enough to wallow out just a bit if the bearing starts to go out. An uneven or loose seating can cause premature failing.
If the wheel/bearing assemble slides onto a shaft, then check to make sure the shaft is not worn or bent just slightly, can also cause premature failure and/or bad tracking problems.
If all looks good, make first couple of runs with a new fine grit belt and let some steel or wood just rest against the belt. You should be able to feel any hopping or vibration...if none you're good to go. If there is some, find and correct before ruining the new bearings/wheel.