Running account balance

First thing I would do is scroll back to the very beginning of the account register & see if you have some sort of Live Opening Balance transaction that's throwing things off. Quicken will sometimes add a transaction at the opening date of the account to adjust the account balance to what it think is the correct value, and it doesn't always get it right.

If you fix that & the running balance is still off, I would start comparing the Quicken balance to your old statement balances. Start by checking once a year - maybe the year end balance each December 31st - to narrow down where things start to go wrong. Once you've identified the year where the error occurs you can further narrow it down, checking the balance at the end of each quarter, then each month, until you find the source of the error.