Engine Timing wont stand still

1st place I would look would be at the timing chain gear key way...or the oil pressure tension slide bar for the chain...??? something is moving and is not tight...

Celica has a 3SFE engine has no timing chain,has belt instead. when I hook-up a timing light on it, I short -out the" E1 and T " terminals then I move the distributor see the timing mark goes back and forth does not stay put.

Yeah the belt is most likely stretched, has it been replaced?