Yeah I'm not sure what it was. I got a replacement rear camera cable, replacement front camera module and the issues persisted.
Nah, the DR750 is 1080p only, but it is 60fps unlike the DR900. For being able to make out detail in fast moving footage (i.e. a number plate of a car passing you in the opposite direction) the 750 holds the upper edge. the DR900 is 4K 30fps I believe, so it trumps the 750 in terms of overall video quality but isn't 60fps so it sort of loses an edge when it comes to trying to make out detail in fast moving footage. I do believe the 750 is slightly better in terms of night time quality as well. One thing to note though is if you want built in voltage monitoring you need to get the X series, otherwise you'll have to get a Power magic pro or similar to do that part for you :). Theres also LTE versions of those cams (I believe Australia is on the list of supported countries) if you want, so you can use the cloud features wherever you are, even if you aren't with your car.
Could go on and on, they are expensive, but I do think its a very good package for what you get.