Autofill in Check Register

Downloaded transactions contain a Payee name and may contain Notes, but will never contain a category. However, Quicken has server-based processes to try to clean up Payee names (e.g. to change "Home Depot #1234*Anytown" into "Home Depot") and to assign a category based on the Payee name. You can turn off these server-based operations in Preferences > Connected Services by unchecking the box for "automatically improve the quality of downloaded payee names and categories".

There's another possibility for how Category and Notes can be assigned to a downloaded transaction: if you have had Quicken set to create QuickFill rules as you enter or edit your transactions, a QuickFill rule is then applied the next time the same Payee is used.

With the limited information you provided, it's unclear which of these issues is involved. Is the information you're seeing in Memo/Notes, for instance, really "random", or is it possibly something in your Memo/Notes from the previous transaction for the same Payee? the latter would point toward it being a QuickFill rule.

One way to get a clue is to click on one of your downloaded transactions with a "random" Notes and select View > Inspector. In the bottom third of the Inspector window are several fields under the heading "From Your Financial Institution". Statement Payee is the name of the Payee exactly as your bank is sending it to Quicken, and Statement Memo is the Memo exactly as provided by the bank. Is the Statement Payee and Statement Memo are different from the Payee and Notes fields above, which are what is save in your Quicken register, then the hunt is on to determine if they were changed by Quicken's background server clean-up processes or by the presence of local rules in your Quicken file. I mentioned above how you can turn off the server rules.

To see if local QuickFill rules are an issue, open Window > Payees & Rules and click on the QuickFill Rues tab. Here, you can see (and edit or delete any existing QuickFill Rules which have accumulated). Check to see if some of the Payees you're seeing in your checking register with "random" Memo/Notes are in this list of QuickFill Rules. Double-click on one of the rules, and you'll see exactly what information is saved for that Payee, which Quicken will apply the next time that Payee is entered or downloaded. You can delete QuickFill rules by clicking the minus sign at the bottom.

If you have a lot of accumulated QuickFill rules and if you decide you want to delete them all, click on the first one, scroll to the bottom and Shift-Click on the last one to highlight them all, and press Delete. (You may still have some in the list with a little triangle icon to the left of the name. The triangle indicates there are multiple QuickFill Rules for the Payee, so you need to click the triangle to display the multiple rules. Do this for all remaining Payees in the list, and then you can again click the first, scroll to the bottom and Shift-Click on the last one to highlight them all, and press Delete.) Finally, if you want to prevent Quicken from creating QuickFill rules going forward as you enter or edit transactions, go to Preferences > Register and look at the last item in that screen for enabling or disabling the creation of QuickFill rules. Uncheck it if you don't want Quicken creating QuickFill rules in the future. (If you want QuickFill rules, but only for Category, you can change the seeing from All Fields to Category only.)