Sounds like your electric clutch is going bad. Its pretty simple...pull the switch, which turns on the clutch, which turns the belts. So either the clutch is slipping, or the belts are slipping. I suppose the little spinner on the electric clutch might also be going bad...a worn out spinner might do what you describe, although I usually had them either work or not.
I had this exact symptom on a 9600 this fall, the drive became useless and then in short order would barely engage and running the front end slow and so observed the clutch and sure enough it was slipping. Did some calling to a mechanic and said the brushes within the little housing on the end of the shaft are probably failing. Sure enough, they were wearing and the springs getting weak and clogging up with dust/wear particles from the brushes when the cover was pulled on it to look inside. Bought a whole new brush assembly from JD ( they don't sell the pieces inside of it at JD, no idea who makes it as its not made by JD ) and it was back to working perfectly. I don't know if the 8820 is exactly like the 9600 but got the impression its similar and from what your describing with that useless half hearted attempt at the front end turning over, its very possible its the same culprit as what I had happen. I wasn't the one personally hands on doing the replacement of the unit but from what I saw it wasn't too complicated and just to get the wire ends screwed onto the same terminal spots as before and orient the holder strap so the grease nipple is in a good spot to reach to grease. If you can't get a unit right away it might work to pop the cover and see if there is some brush length left yet ( 4 brushes I believe ) and see if its possible to clean the unit some, stretch the springs a bit and reinsert them but again or shim the springs with a piece of plastic etc, I never pulled the brushes apart in my old one to see if it was possible to fiddle with it to get a little more life out of it until a new one could be installed. Hopefully I am not steering you in the wrong direction, just sounds so similar to my symptom in its lazy engagement and not able to actually combine with it.
Also if you can't get one right now into your hands, you may have a good neighbor who has his combine parked for the winter and can borrow the unit for the time being. That would also tell you if its even the problem or your magnetic clutch itself had failed electrically.