'Upload Media' is unresponsive and typing lags

Since changing web hosts I’ve had all sorts of issues with ONE of my four WP installations, which just happens to be the main and most important site to me.

The issue which is currently unresolved (and sending me mad) is as follows. You can see a video of the issues here.

‘Add Media’ unresponsive on both Chrome and Firefox.

When I click the ‘Add Media’ button, or ‘Add to Gallery’ or ‘Add Featured Image’ the window will open but then it will just not respond. I can’t click on anything, I can’t enter text in the search box, can’t select and image, nor can I drag and drop. I have to refresh the page to get rid of it.

This is stopping me from updating portfolios and adding blog posts as they should be very visual.

My new host has been trouble shooting, and it originally did it for them but now it seems to work on their computers and also on my laptop however it won’t work on my main desktop still. Anyone know why this is as it’s sending me mad – everything I see for the site is on my main desktop.

Lagging type
Reaaaallly annoying when I’m trying to add text to portfolios, pages, posts etc. I’m just typing nicely and then suddenly I’m still typing but the words aren’t showing, the WP freezes for a second, seems to be when it autosaves.

Here’s what I have done so far to try and fix the issues:

? Updated firefox, I was one version behind
? Disabled all plug-ins
? Re-installed WordPress
? Thoroughly cleared browsers cache
? Cleared DNS on my machine
? Had the tech support fix permissions as somehow they were messed up

I don’t know if it helps but after the transfer of the cPanel from my old to my new hosting company I lost a hell of a lot of images from my uploads folder and I was also locked out of my account, log ins wouldn’t work at all even though they were correct, this got solved.

Anyone got thoughts about how to resolve this? It’s sending me nuts!

Thanks!