Check all connections between PC and monitor to see if they are tight and/or you could try another cable if you have a spare. You could also check your display settings in Control Panel. In Win7 they are accessed via Display - Advanced Settings - Adjust Resolution (properties) - Screen Refresh Rate. You should find something similar to that for XP. Adjust the Refresh Rate - Apply - OK to see if that makes any difference.
OnePlus 5T review: Hands-on