Random Shutdown On Studio 1555 Using Ubuntu

I have Ubuntu 20.04 installed on an old Studio 1555 and it keeps randomly closing down. I've changed the battery and ram and I'm using an external mouse. I bought the laptop second hand with the Ubuntu already installed. I realise at 4GB of DDR2L of RAM I'm at the limits of my machine. It won’t let me install a lighter Linux as it says it can’t find the 'live files'. I’ve tried several version using usb pens written on both my Windows laptop and my iMac main computer.

Any help would be appreciated as I’m determined not to be beaten. Thanks everyone.