The Allison Transmission is mechanically practically bullet proof. I had a similar situation, with some really weird symptoms, and it ended up being the Transmission Control Module (TCM.) GM will charge about $900.00 to change this out. If you look around, on a national on line retailer (starts with the letter "A"), you can find this part for around $285.00 whereas GM will charge $700.00 for the same part. It will need to be programmed by the dealer. Might be worth having a spare one around as these are known to cause similar problems.
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