Canon 550d not working properly with card in

Hi, really need some help as my canon 550d isn't working properly.

Here are the basic facts:

Recently photographed a wedding- took a few thousand pictures (swapping between 2 cards). I was using external flash extensively for the first time (This is probably not related but you never know).

Relatively soon after this (whether it is related or not) the camera started going wrong. I was set to record video for a short i'm making, and suddenly, the liveveiw keeps shutting itself off (the shutter physically closes too) This happens in video mode, photo liveveiw, and has also happened in play mode and on menus (but not standard photo mode without liveveiw, as this is what it switches back to).

I am using Magic Lantern, but removing magic lantern didn't help. Curiously, the camera works fine under 3 conditions:

A: Without an SD card in the slot
B: When plugged into computer via USB
C: Sometimes if left for a while, it starts working again, often for several days, getting my hopes up- then failing again.

When it fails, it quickly starts doing it more and more until eventually the camera becomes unusable. It even freezes the whole camera with BUSY and won't let me do anything (ML's A1ex has provided a solution to this problem, so I can stop it freezing up with ML firmware, but the original problem remains) We have tried all the usual solutions and some more unusual ones- We used a hidden factory reset that restores several default settings, including the setting the language back to japanese. I thought this had worked at first, but it turns out it hasn't.

What really confuses me is how it works fine under those conditions (which are useless for actually using the camera).


If anyone has any suggestions or possible diagnosis's, then any help is very much appreciated!

Thanks

Josh

(PS. have contacted canon but haven't had a reply yet. But I'm not expecting them to be much help, and my camera is out of warranty and was imported anyway)

To clarify, the camera does work with an sd card in when it is plugged into my mac via usb. in this situation, everything works perfectly and I don't need to touch the computer's software to do it.