Has your Memorized Payee List grown too big over time?
How many Memorized Payee List entries do you have in your file? A couple hundred? Several thousands? Click Help in the Menu Bar. CTRL+click About Quicken and it'll tell you in a File Information window.
To trim the list and keep it in shape please do this: In Edit / Preferences / Data Entry and Quickfill make sure you have a checkmark at[√] "Remove memorized payees not used in [xx] months" and set [xx] to something like "18". Do not set it to zero. IMHO, you should set the number to 12 or higher for best results, so that once-a-year payees don't vanish. Restart Quicken for the change to take effect and housekeeping to clean up the Memorized Payee List. Check the File Information box again to see the reduced count.
If you have multiple MPL entries for the same payee, delete the unused or unwanted ones. You can set the "Lock" option on MPL entries that are properly categorized. This will prevent a new transaction from changing the MPL entry to something unwanted . If you regularly need multiple MPL entries for the same payee, but different categorizations, use distinct Payee Names, e.g., Costco Wholesale, Costco Gas Station, etc.
This might be worth a try …
Rebuild MPL
After creating a backup of your Quicken data file … To rebuild the Memorized Payee List from existing transactions, click Tools, hold down Shift and CTRL, then click Memorized Payee List.