The timing mark is on the outer ring of the harmonic balancer/combined with the crankshaft vbelt pulley. If the outer ring moves relative t¿ the center ring (the elastomer ring between the two is yielding or the bond is failing) the  iming mark will be off and/or impossible to find or maintain.

Scribe ¿r paint a white line across the elastomer ring so you can see if the line is broken after a few days or even when you rev the engine a bit. If so, replace the balancer. Not unusual to replace them every ten years or so...I went through a couple in 150K miles on the 87 760T.