[RobNottsUk]

Perhaps stack size effects come into play. When you're one of the shorter stacks, then loose opponents are denied the implied odds they need to draw out. When your stack becomes deeper, the leverage of future betting means your odds are worse on the flop than they appear, particularly if these players have gained a reasonable idea of the kind of hands you play, and now due to respect of deeper money pay off less.

Perhaps you should try buying in for less than the maximum for a bit, and leave the table once you've doubled up. The odds are, that eventually you lose a large pot, so if several players cover you several times over, the luckiest out of a pool of poor players, you become a favourite to loose your winnings at some point.

So perhaps it's not just fatigue, and tending to leave the table when you're fed up and become stuck, but actually not adapting your play to changing stack sizes (pf raise sizing, and money remaining on the flop).

The PLO book by Slotboom, has nice discussion of varying play to stack sizes, though I'm not a PLO8 regular player, much strategy of PL play must be common to both games.