MBP powers down after 4 seconds (part 2)

In December I received great help from this site in relation to a MBP Model A1211 which would not power up properly. After trying all the suggestions and replacing a faulty fan with one previous ordered from iFixIt, the MBP worked again for all of 5 days. On days 6-7 the screen would suddenly go black while the laptop was in use. From day 8 onwards, I have been trying various tests and strategems, chiefly as suggested in answer to the "MBP powers down after 4 seconds". I once got the laptop to stay on for 5 minutes, long enough to retrieve the 10.6 installation disk from the drive, but that is the record time so far.

If the faulty fan has been replaced, where should I be looking next to get this laptop working again? Graphics card? Logic board?

Thank you in anticipation.