Typically, everyone I've helped with the intermittent lag spikes, or what seem to be "freezes" where your ESO stands still for up to 5 seconds, have been caused by the same issue.

It seems the freezes are a result of poor management by multi-core processors. To check this out, hit Cntrl+Shift+Escape to bring up your task manager, and switch to 'performance'.

The next step is much easier on a dual-screen setup, since you need to be able to monitor the performance tab of your Task Manager to see if your processor is poorly managing resources (which has been the case 6 times thus far). Either way, you must be able to see your Task Manager while the game is running, else you can't verify if this is the issue. So put the task manager on your second monitor, or run ESO in windowed mode for the duration of our test.

Basically, somewhere in the Windows OS, resources are being completely diverted to core 1 while playing ESO. This almost always causes it to cap and overflow into your 2nd, 3rd, 4th (and beyond) cores. The problem in overflowing, is it can cause your computer to practically "give up" on trying to manage the resources, hence your huge lag spike.

To fix this, start by doing a clean restart before you start playing ESO. This means exiting all your programs, and using the start menu to enable a restart. Let it safely shut down all processes, and restart. I'm not 100% sure why, but this has fixed my issues, and a handful of my guildies issues.

TL:DR; Windows manages ESOs resources poorly unless it's started on a clean boot. Restart your computer before you play ESO, and your lag should be at a minimum. You should also reduce your load by turning down some intensive options (like draw distance)

Make sure to drop a line if this worked for you.