IF THIS IS A SEPARATE ROKU DEVICE, not a TV with Roku functions built in:
One thing that MAY help is setting your Roku to your TV's actual resolution instead of it's auto display setting.
Settings > Display type
When you change the Display type to a new setting it runs you through several screens:
A screen where you can choose from the available options on that device.
A screen where it analyzes your choice to be sure your tv supports it, then gives the option of trying that choice if you wish.
A screen with a countdown timer and two choices: Yes, screen looks good No, I will choose a different setting The "No" option is highlighted by default. To keep the screen resolution you're currently trying, you need to arrow up to the "Yes" option and click on it before the timer runs out.
Another thing to try, if your Roku has this setting:
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X