@Tfraley, to be completely honest the 5k imac is very much underpowered for handling any modern and moderately demanding (or more) game at its native resolution since it only has a mobile equivalent of a radeon r9 290. 5k is literally around 70% more pixels than 4k is, to give an idea here. Your best options, in my opinion, would be:

-I'd try this for sure before anything else: in your ESO "usersettings.ini" file, change the line GPUSmoothingFrames = 10, to a 0 (zero) instead of 10 (ten). That setting asks the client to do a larger-than-normal frame buffering amount, which if your framerate is very high won't be too noticeable except for the added input lag, but if you're on the cusp like you are, it can result in a very unpleasant experience. Most games do double or triple buffering which is 2 or 3 respectively.
-Enable subsampling to "medium" inside of ESO. It'll still look very nice but should ensure you maintain a strong framerate by rendering at a lower resolution than the 5k native and upscaling it.
-Lowering your settings significantly is another option.

Good luck and shoot an update later on .

OK,

Here is some more info.

First the description of jittery.

Say I am running through town, it doesn't feel smooth.
It's like he is skiping.
ie background moves like every second (I am probably over exadurating but its how it feels)

As it currently stands I get anywhere from 14fps to 45fps its all over the place.

Here is a copy and past of my current config.

SET MouseRawInput "1"
SET MouseSensitivityFirstPerson "0.50000000"
SET MouseSensitivityThirdPerson "1.00000000"
SET MouseSmoothing "0"
SET InvertMouseWheel "1"
SET InvertMouseY "1"
SET InvertMouseX "1"
SET MAGELIGHT_LIGHTS_VISIBLE "1"
SET PFX_SUPPRESS_DISTANCE_v2 "46.00000000"
SET PFX_GLOBAL_MAXIMUM "831"
SET MULTI_GPU_CHECK "1"
SET CachedRLREnabled "0"
SET CachedReflectionResolution "0"
SET CachedShadowFiltering "3"
SET ScreenshotFormat.2 "PNG"
SET PregameGammaCheckEnabled "0"
SET HardwareCheckEnabled.3 "0"
SET SUB_SAMPLING "2"
SET REFLECTION_QUALITY_v3 "0"
SET PARTICLE_DENSITY "2"
SET HIGH_RESOLUTION_SHADOWS "1"
SET SHADOWS "1"
SET GAMMA_ADJUSTMENT "123"
SET VIEW_DISTANCE "1.00000000"
SET GPUSmoothingFrames “10"
SET MIP_LOAD_SKIP_LEVELS "1"
SET RAIN_WETNESS "1"
SET LENS_FLARE "1"
SET GOD_RAYS_v2 "1"
SET ANTI_ALIASING_v2 "0"
SET AMBIENT_OCCLUSION "1"
SET COLOR_GRADING_v1 "0"
SET BLOOM "1"
SET DEPTH_OF_FIELD "1"
SET TONE_MAPPING "1"
SET COLOR_CORRECTION "1"
SET SOFT_ALPHA "1"
SET DISTORTION "1"
SET CHARACTER_LIGHTING "1"
SET DIFFUSE_2_MAPS "1"
SET DETAIL_MAPS "1"
SET NORMAL_MAPS "1"
SET SPECULAR_MAPS "1"
SET CLUTTER_2D "1"
SET SIMPLE_SHADERS "0"
SET WATER_FOAM "1"
SET VSYNC_INTERVAL "1"
SET VSYNC "0"
SET MAX_ANISOTROPY "2"
SET PreferMaximizedWindow "0"
SET PreferExclusiveFullscreen "1"
SET FULLSCREEN_v5 "2"
SET FullscreenHeight "1440"
SET FullscreenWidth "2560"
SET WindowedHeight "1330"
SET WindowedWidth "2560"
SET FOVChangesAllowed "1"
SET AgreedToPEGI "1"
SET VoiceChatAutoJoinChannels.2 "0"
SET VoiceChatEnabled.3 "1"
SET LuaMemoryLimitMB "64"
SET GamepadSensitivityFirstPerson.2 "0.72000003"
SET GamepadSensitivityThirdPerson.2 "0.85000002"
SET GamepadInvertX "0"
SET GamepadInvertY "0"
SET GamepadVibrationEnabled "1"
SET SOUND_ENABLED "1"
SET FOOTSTEPS_VOLUME "50.00000000"
SET FOOTSTEPS_ENABLED "1"
SET SPEAKER_SETUP "0"
SET BACKGROUND_AUDIO "1"
SET VO_VOLUME "92.00000000"
SET VO_ENABLED "1"
SET UI_VOLUME "70.00000000"
SET UI_ENABLED "1"
SET AMBIENT_VOLUME "70.00000000"
SET AMBIENT_ENABLED "1"
SET SFX_VOLUME "70.00000000"
SET SFX_ENABLED "1"
SET MUSIC_VOLUME "85.00000000"
SET MUSIC_ENABLED "1"
SET AUDIO_VOLUME "67.00000000"
SET AUDIO_ENABLED "1"
SET TooltipFilter "-1"
SET TooltipNumOptions "2"
SET Language.2 "en"
SET GraphicsDriver.7 "OPENGL"
SET PatchDataPath "game:/GameData/"
SET CacheDataPath "machine:/Cache/"
SET ViewedAddOnEULAVersion "5"
SET AcceptedAddOnEULAVersion "5"
SET AcceptedEULAVersion "3"
SET DirectionalDamageIndictorVisibleInThirdPerson "0"
SET CameraFramingTransitionEnabled "1"
SET SkipPregameVideos "1"
SET HasPlayedPregameVideo "1"
SET RangedKillCamera "0"
SET PlayerKillCamera "1"
SET LastMonsterKillCamera "1"
SET MinFrameTime.2 "0.01000000"
SET ParticlesEnabled "1"
SET RequestedNumJobThreads "-1"
SET RequestedNumWorkerThreads "-1"
SET FreeUnusedGrannyFileSections.2 "0"
SET IsTrustedMachine "1"
SET RememberAccountName "1"
SET AccountName "XXXXXXXXX"
SET LastRealm "NA Megaserver"
SET CustomUIScale "0.80813199"
SET UseCustomUIScale.2 "1"
SET RollDodgeDoubleTapEnabled "1"
SET RollDodgeDoubleTapTimeWindow.2 "185"
SET ClampGroundTargetEnabled "1"
SET MonsterTellsEnabled "1"
SET CameraZoomDistancesSynced "1"
SET SiegeCameraZoomDistance "6.00000000"
SET WeaponsOutCameraZoomDistance "5.00000000"
SET WeaponsSheathedCameraZoomDistance "5.00000000"
SET MaxNetworkWriteBuffer "10485760"
SET MaxNetworkReadBuffer "10485760"