It is normal behavior when the spring is unwound. Interesting reason why as well!
With a chronograph, lets say the power reserve is 42 hours. At 42 hours from last wind, what happens is the friction, or drag of the movement, overcomes the amount of torque that remains in the mainspring, at which point the movement stops. Lets say the amount of torque in the mainspring required to run the movement at this point is 1.0.
Now, if the chronograph is running, more gears are in motion, and the drag and friction of the movement is increased. This means it takes more torque in the mainspring to keep the movement running than it would if the chronograph was disabled. So lets say the torque required to keep the movement running with the chronograph engaged is 2.0.
Now at the point 41 hours after it was fully wound, the torque in the mainspring is, lets say 1.5. Now that means it is NOT enough to turn the chronograph as well as the normal movement, but it is enough to run the movement on its own.
So you were engaging and disengaging it in that grey area, near the end of its power reserve, causing it to stop when the drag became too high (engaged) and start when the drag was reduced (disengaged).
Give it a full wind and enjoy your watch my friend, all is good!
Post a picture too when you get a chance, Speedmasters are ALWAYS welcome here
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