However, these numbers can jump around. If this happens then when I call the next group of transactions (129000 to 128000) transaction 129000 may not be the same as in the previous call.

No thats inaccurate. Transaction numbers should be the same on every call. If you start at trx 12000, it should start at the same place every single time, since new transactions are added at higher indexes and you only go towards lower indexes.

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