What you are describing sounds like a wing stall caused by too much elevator travel, applied too quickly. Try applying the up elevator slowly and see if the problem goes away.

You should not have to add 7 ounces of lead to the nose to balance this airplane. An extremely nose-heavy condition could account for the difficulty in getting off the ground and also the stall at full elevator travel.

I suspect something is wrong with your balancing method. What do you use as a balancer?

Jim