I would try flipping the jet vents the other direction.

From your description, it sounds as if the rear is bottoming out when hitting a bump. You have the stock (soft) springs on the rear, with 40 weight oil, and the pistons set up for fast upward travel. When accelerating, the rear shocks are probably already 3/4 of the way compressed, and just the smallest bump would cause them to bottom out, and the rear end bounces because of that. Since the rebound resistance is more than the impact resistance, hitting many bumps in succession is also probably causing the rear shocks to "pack up", which only adds to the poor performance.

I'd try flipping the jet vents, and putting heavier springs on the rear. Try it out, then you may even need heavier oil.