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I will try to help.
Are you saying it will always start in neutral?
It more than likely is the starter. When you bench test it is not what we "heat soaked" and could test fine.
If all battery, cables, etc. are ok. I think your solenoid is bad. I would have it tested by a shop that rebuilds the units themselves if you have one in your area. You will get a better unit that way in most cases as well. Also auto electric shops can tell you if it's a common failure to that particular starter. I can tell you that it's a common symptom of almost all GM starters.
I dont think it has one. You can test for power at the small lead at the solenoid when the problem occurs with a test light. Should have power with the key in the start position, if it does, and it wont start, the starter is bad.