That laptop is getting up there in age (around 10 - 11 years) and I suspect the screen is showing signs of that age. The fact that an external monitor (with a faster refresh rate) does not do it would seem to indicate an LCD laptop screen problem. Try running system diagnostics. Restart the system and at the Dell splash screen start tapping F12. Run full system diagnostics paying attention to any LCD errors. But possibly the time is coming to consider a new laptop with far better performance and a better display.
This may not apply but it is worth noting