Revs drop very low when electrical are used

Whenever I electricals such as the ac, electric windows, windscreen wipers, brake lever (brake light) etc I find a significant drop in revs, sometimes enough to almost stall the car if I put the car under significant electrical load (using lots of these at once). When this happens the revs drop very low, then start coming back up if they hit around 200 revs, this is particularly noticeable when driving and coming to a stop (brake light on from brake lever depressed) as the car can sometimes almost stall as coming to a stop...

Whilst trying to diagnose the issue and tune things up I've removed the idle air control thingamajig and cleaned it out nicely and checked and cleaned the pcv valve, also the car has a new battery and alternator...

Could anyone suggest any solutions or further things to check? I've struggled to find much info on this issue and I'm not very familiar with cars so I'm very much still learning!

Thanks!

So I had a look at the earth cables and noted they were pretty filthy at the connection points, some of the cables looked damaged in places too so I have cleaned up the connection points and replaced the cables.

Now I have a new issue, the car isn't almost stalling as I come to a stop now, Instead when I'm coming to a stop the revs shoot up when I press in the clutch, revving between 1500-2000, calming down after 10 seconds with the car in neutral and my foot on the brake or handbrake on...

On a side note, if the car is sat at Idle and I hold the button to close the car window (when it's already closed) the revs still drop pretty sharply, but it can seem to handle other high electrical load at Idle (headlights, indicators, and wipers all running together)...

Thanks for your help so far!