I think the aluminum frames are bit too stiff for your weight.

have you considered talking to a frame builder about the situation that is going on?

now that you say you broke 2 frames its something to consider.

as for fixing I had another thought.

as you have nothing left to loose in this frame and it is highly likely to happen to your next frame....

find an aluminum welding specialist locally (not hard to find at all)

or ask a frame builder about heavy duty alloy lower tubes, and explain why they are broken-

the 2 lower tubes can be completly removed and new ones built in-stronger then before.

send a pm to frank the welder about this