Problem with my Stevens 520, disconnecting pump handle!

[ComradeQuiche]

Hello all,
I see there is quite a good bit of info on the Stevens 520 here!

I got one fro a family friend, stripped it down, cleaned it, and shot about 100 rounds through it during a trap shooting competitions.
Worked flawlessly, fits me like a glove.

BUT

Then I tried shooting doubles instead of doubles.
2 birds fly out of the house, I shoot at one, rack the pump back, and when I pull it forwards again suddenly there is no tension! The pump handle has suddenly disengaged from the rest of the gun!

I try it on three more occasions, shooting singles it works flawlessy, BUT when pumping more than one shot through the gun the pump handle becomes disconnected EVERYTIME! I am guessing it is a warn part? Any ideas?