Who made the timing set used?
Which tensioner did you use? The all steel unit with the ratchet or the plastic unit?
Did you inspect all of the roller followers for loose or worn needle bearings?
Did you replace the oil pump? Has the oil pressure been measured?
How many miles on this motor? IMO considering how many times this major job has been done without success you should perhaps consider that the reason for the problems might be low oil pressure. What could CAUSE low oil pressure?
- debris in the oil pan. When the original timing set failed, did any the broken pieces fall into the oil pan?
- worn main bearings. Excessive clearance will cause low oil pressure.
- debris or slug in the upper cam head passage ways.