I know there is a better way to do this, I just can't remember what it is. Something like this should get the result you want I believe.
Code:
for elm in arcpy.mapping.ListLayoutElements(mxd, "TEXT_ELEMENT"):
if elm.name == "ATSList":
elm.elementPositionX = 6.5
elm.elementPositionY = 1.0
entry = "\r\n".join(FieldValList)
elm.text = "Within Theoretical:\r\n{0}".format(entry)
Edit: Maybe that is better
Edit2: If they are not string values in your list try this.
Code:
"\r\n".join(map(str, FieldValList))
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