Check the condition of the auger flighting where it enters the bin. If it's badly worn or even folded back a bit it will have lost it's push. If there's a leveling auger in the bin check that also and check it's position relative to the outlet and make sure it is actually turning. (this is from memory when we used to run G's) Also check the divider board in the bottom of the clean grain elevator, they do wear out ... Also check the roll pin in the sprocket at the bottom of the elevator..it may be broken and just driving by the set screw and slipping when the load increases.
Common problem on F-2's in corn and the wetter the corn, the worse it is. I always used to throw away those little sliding adjustable augers and replace with full diameter auger flighting from the elevator out to the cross brace in the bin (about 18 to 24 inches) and cover it with a metal tube. Bin will fill more evenly and slip clutch issues will subside. I'm not saying the slip clutch doesn't need some increased spring pressure too, but changing the bin loader auger design is a real positive step to solving the problem.