You should not be bogging due to hp requirements of unloader. Check fuel flow as in fuel filters and air filters. As per posts you might need to add a flap or fingers to center of auger.
as for the light coming on, check sensor and bearing collar with tabs. Check wireing Make sure the upper drive pulley is not wallowing out the spline on the shaft. This lubes through the slip clutch on some and the shaft end on others.
Be sure auger has flighting that will intersect a common line around tube. We have done this to most all heads for the building up problem. We have used up all the left over flighting from narrowing an 830 to 822 so will have to go to parts book to get flighting know. If I can't get flighting that way I will order the bolt on flighting from old corn heads and make that work. Dry leaves seam to cause this problem more than anything but still I'd extend flighting. After checking out sender clearance ect. for rear elevator light slowly raise and lower RPM to see where light goes out and or on. Module in overhead has a tweaker screw on it to adjust what RPM light comes on and or off. Tweaker screw is on side of module in proper relation to lights. Chopper had two holes so light can be adjusted when switch and belt are set for high speed and low speed. Use tiny screwdriver and don't force the screw beyond its stop. Otherwise check for loose wire at plug in overhead or inside fuze door for if rear feed was acually slowing up the front feed would certainly be pluging up quick. Oh yea check that belts are staying tight. Check gap between sheaves of corn varible if running slowed all the way up. Check right front belt on feeder to see that sping is set to length of tin along spring or maybe a bit tighter and be sure that belt tentioner to rear of that belt can still swing to keep belt tight of it pulls. I have seen some of them rear feed belt tentioners bottom out in pivot tentioner pivot area thus preventing belt from staying tight enough under load. If any of them belts are sliping you would likely plug but it could be it is just slowing up a bit and light happens to come on at that point. Good luck