I would say it has to be water related. Something of weight is moving around, and I can’t think of anything else it could be. Without knowing the design of the boat, I would hazard a guess that there’s water in the hull and it’s moving forwards and backwards as you increase speed and decrease speed. However, there’s probably a blockage in one of the central sidewards drain holes at either the aft or the stern, so that when the water runs either forward or backwards, it can’t equalise side to side and level the boat, but once it runs back it can once again equalise. Just an idea.