Exclamation [Q] Download mode appears to not work? (Custom ROM)

I'm trying to flash my S3 International (i9300XXUGMK6) to Cyanogen Mod.

The phone currently is running stock jellybean but was rooted some time ago with SuperSU.

First i tried the installer from the CYMod site, all went OK until "Flashing recovery software". The phone went into download mode but eventually the laptop times out saying unable to connect to device. Phone needs rebooting to get control back.

Second time i tried http://wiki.cyanogenmod.org/w/Install_CM_for_i9300 instead.

Again instructions worked fine until the phone entered download mode. No "ding dong" USB noise from the computer and Heimdall exits with "ERROR:- Failed to detect compatible download mode device"

It seems at least as in download mode my phone wont "talk" to anything.

I've tried 2 different computers (laptop and desktop) and 2 different USB cables.

The phone works as normal in USB mode when not flashing ( to copy photos, view directories etc).

I've tried to redownload a stock firmware but i cant find a single working link now - they all use terraserver which 404s. I'm not even sure i can flash anyway as ODIN wont recognise the phone either with the same drivers off its site.

Any idea how to fix download mode or get it working so i can flash?