Voltage surges or flexes after running good fifteen twenty minutes

Okay, some Jackie replaced the harmonic balancer now when it's running you see the pulley off the crankshaft going in and out like majorly out of alignment . Though it runs okay fifteen twenty minutes then you lose power like voltage to everything lights get dim rpm drops like battery is draining or not holding charge. Battery checks fine installed new alternator it tends to do it much faster with AC on . Did start to squeal but does not after changing alternator . Any suggestions would be helpful please .