Old congealed oil and grease can slow down the movement of internal parts, thus now allowing timely engagement or allowing partial engagement between sears and tumblers.
My Browning OU presented this problem. THe maddening thing is that once the sun was up and the action warmed, causing the oil to thin out the problem went away.
A good degreasing and careful inspection of parts should reveal the problem. Since this is an action not maintained for 5 years it seems that the cause could well be the build up of old lube and perhaps corrosion along with weakened springs.
