MK1 power disappearing on start up

[Pro car crasher]

Hey everyone, so my mk1 runs pretty well most of the time but I've been having a weird issue with the electrics when starting the car. It doesn't happen very often, just happened a couple days ago and the last time it happened was well over a year ago. Basically when I turned the key all the usual stuff lit up and then as soon as I turned to ignition there was a click and suddenly everything goes dead almost as if there was no battery connected at all. The only sign of a charge is that after a while of leaving it alone, the LED light on the immobiliser starts to flash, starting off very dim and eventually getting brighter. Nothing else seems to work except the horn which will just about make an audible noise.

I left the car sat for about 2 days and this morning tried to start it several times with a jump pack with no luck. I thought maybe there was an issue with the battery so I went out and bought one, when I got back I thought I'd just try and jump it again and miraculously it fired right up.

My main suspicion is a short somewhere but the way it seems to suddenly lose all charge and then slowly regain it just seems very odd to me. I'm terrible when it comes to electrics so not sure where to start looking, part of me wants to just try redoing the whole wiring loom to rule out everything but that would be a hell of a job so would be a last resort.

If this sounds really obvious to anyone or sounds like a problem you've had before I'd appreciate any insight on what it could be.

[Belle427]

Check the earth cable in the boot is clean and still secure on its bolt next to the battery.