Well that's unfortunate, sure felt the spring would be the culprit. I still feel that the transmission is seeing more throttle pressure than is really there. First I would double check that the control rod is adjusted as short as possible and also that the spring on the detent at the bellcrank is holding and not letting the linkage slip into passing gear mode when you press the accelerator. If all that checks out then I think I would next take a look at the valve body.
IMG_0939valve body 2.jpg
valve b0dy 2.jpg
the valve body on the right in each of these pictures shows a stuck throttle position valve. When working correctly the control arm rests against the piston behind the L bracket. Foot pedal movement will push the piston into the main body where it compresses a spring, visible in the open area in the middle of the body. Once spring is compressed enough it will move a second piston to exit the other end of the passage. Once foot feed is lifted the second piston should be snapped back into the main body by the spring steel strap. If either the first piston is stuck in, or the second piston is stuck out then he transmission will be seeing incorrect throttle position. Valve body on left shows pistons in correct position when at rest.
Not saying this is the answer to you're problem, just where I would look next. If all that checks out OK then I think I would do an air check for a broken distribution ring. Pretty easy to check while the pan is off, but not so easy to repair if needed.