Why is my 1986 Jeep Cherokee is destroying serpentine belt?

I have replaced the alternator with a bone-yard unit. The belt will sometime run half way off the fan pulley, and other times, it will run off the back of the harmonic balancer. It is the 2.5 with power steering and air. It appears that all the pulleys are in line, except for the harmonic balancer. I have done a lot of engine work recently, to include, head gasket, timing chain, water pump, clutch fan, thermostat, ignition controle module, spark plugs, wires, distributor cap, rotor on,and intake gaskets. I have checked the tightness on everything, and all is tight. I need to do the motor mounts next, but want to get this fixed first. Any and all help will be greatly appreciated. THANX