my external monitor keeps flickering

In Sys Prefs>Displays, hold option key while selecting Scaled for that Monitor, try other resolutions.

What Rez is the Monitor & what exact cable are you using?

Could be a bad cable

For instance some Higher Rez displays won't work well with cheap 30fps cable, Make sure it's like 60fps for 4K.

Good to hear, I think we're on the cure... what connections does the hp have?