Electrical Gremlins

[Bobario]

Hi all, I put 2 new batteries in my car recently main and aux. The car ran flawlessly for 2 days including a 150 mile round trip to Derbyshire and back. It was then left unused outside my house for 2 days. The next time I tried to use the car I got 10 yards down the road and noticed I had no Rev counter or speedo.

In short, and only while the engine is running, the car will throw all sorts of warning lights including brakes, srs airbag, coolant system and others. The engine starts/runs fine, pas, brakes and handbrake. Indicators work but no clicking noise and flashing arrow on the console to tell me it is on. The radio and ac and anything else on the centre console only works intermittently, as do the electric windows. For a couple of minutes the car will run fine, then the warnings will come back and the speedo etc go to 0. The radio almost never works unless the engine is off, then it runs fine. Unfortunately, the stop/start function is now working fine!

I'm tempted to hand it to an auto electrician rather than my local Volvo specialist who wanted to charge me £102 just for a scan.

Any ideas? Sorry for such a wall of text on a first post but this has me tearing my hair out. I've read through over 240 pages of this forum and many on others and couldn't find a definite solution. I don't want to give it to a garage who may start throwing parts at it when it is just an electrical error somewhere!