This sounds like a loose common ground somewhere. There are multiple grounding points on the car (not just the battery) and I've seen people who have found ground leads that have either corroded through (most common--especially for those in/under the engine bay) or loosened to where they are not reliable any more.

 

This. A very common one.

 

gonna have to have a look I reckon

 

think of it as an opportunity to bond with your car

 

Indeed sir!

 

There's a ground point in the front right wheel well where folk on the UK forum have found corrosion issues.

 

I've only had two electrical issues. The first was a loose battery terminal. The pinch bolt was tight but it wasn't clamping to the post. I needed to expand the terminal a bit so it would tighten properly. The second problem I had was from rodents chewing on the wiring going to the antenna for the passenger side keyless entry antenna. I hope you don't have that problem (rodents, that is).