I would remove all the grounds and clean to bare metal with emery cloth and apply dielectric grease to them. If the wiring harness uses the round AMP CPC connectors make sure the blades and sockets are making good contact. Use electrical contact cleaner to get the dirt out. If you still have the same problems then check the wiring harness for rub through somewhere.
The neighbors TW 35 was acting up with electrical gremlins and it ended up being the CPC contacts in the connectors under the hood where you would split the tractor. The sockets were stretched from vibration and would not make decent contact to the blades. I bent them in with a thin blade screwdriver to tighten them up. This was 2 years ago and its still working. These can be replaced also as AMP (tyco) sells an extraction tool and replacement contacts available from Mouser or Digikey.
If your harness uses a large blue rectangular connector those are Positronics connectors. They are junk in my experience and check those for problems also. I noticed our 8970 uses them and I cringed when I saw them......
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