"I'd guess your larger tires are raising the tongue to where it doesn't have enough pressure on the ball until you load it."
that's what I was wondering, but I do know that there is a little bit of upward rake (the tongue has a slight upward angle to the ball) which is better than downward, correct? also, the frame is super flimsy angle iron...not sure if it's original or not. so I like your idea of building a new frame with a heavier tongue.
J.