Some of Us old gen 4 owners have had this problem i even had a brand new starter fitted and after 4 days had the same problem i had a relay mod put in by my mechanic down the road from me who does work on my cars and its been ok forwell over a year now and been fine fingers crossed , for us its due to i wire from tje ignition to the starter not holding the current as the wires degrade with age so a relay is put in to assist this generally , the mod on mine however is s little differen as the mod is connected to the starter and the battery but as said its worked out ok steve gillies from adl racing can supply these kits as a direct wire on mod for the 3sge or 3sgte engines
Hope this helps