Originally Posted by
Six jours
Honestly, it really does seem to me that the seatstays are being spread/bulged/sprung which is causing the dropouts to move apart. I strongly suspect that the frame is simply not strong enough for the OP. Perhaps the gentleman can hold a straightedge against the seatstays and see if they are perfectly straight.
I've built and have had a number of frames with seat stays which weren't perfectly straight. A slight dog leg at the bridge is pretty common. Besides the seat stays are much less stiff or strong compared to the chain stays. Andy.