SET gxResolution "1920x1080"
I use that and there's no issues you're describing.
Copy of my config, if you wish to see if it helps at all. Make backup of your own 1st ofc.
Code:
SET locale "enUS"
SET realmList "logon.warmane.com"
SET hwDetect "0"
SET gxResolution "1920x1080"
SET gxRefresh "144"
SET gxMultisample "8"
SET gxMultisampleQuality "0.000000"
SET videoOptionsVersion "3"
SET textureFilteringMode "5"
SET movie "0"
SET Gamma "1.000000"
SET readTOS "1"
SET readEULA "1"
SET showToolsUI "1"
SET accounttype "LK"
SET Sound_OutputDriverName "System Default"
SET Sound_MusicVolume "0.40000000596046"
SET Sound_AmbienceVolume "0.60000002384186"
SET componentTextureLevel "9"
SET shadowLevel "0"
SET farclip "837"
SET specular "1"
SET groundEffectDensity "40"
SET groundEffectDist "140"
SET environmentDetail "1.5"
SET projectedTextures "1"
SET weatherDensity "0"
SET ffxGlow "0"
SET ffxDeath "0"
SET gameTip "100"
SET mouseSpeed "0.94999998807907"
SET screenshotQuality "10"
SET uiScale "0.71111111111111"
SET useUiScale "1"
SET Sound_EnableErrorSpeech "0"
SET Sound_MasterVolume "0.5"
SET Sound_ZoneMusicNoDelay "1"
SET Sound_EnablePetSounds "0"
SET gxFixLag "0"
SET maxFPS "144"
SET gxCursor "0"
SET Sound_EnableSoundWhenGameIsInBG "1"
SET gxTripleBuffer "1"
SET realmName "Icecrown"
SET Sound_EnableMusic "0"
SET lastCharacterIndex "9"