if the brushes look uneven like the pic you might have a rough slip ring. Ive resurfaced them before and run for years, you can buy the slip wrings and install them or have an alternator do that. If they are rough the new brush will not last long.
I had similar issues before and replaced my alternator with another known good one I had onhand. I then found that the ground to engine block was a bad contact. it was under the waterpump, in that area.

you can quickly take a pair of jumper cables and carefully double up such connections while checking with a meter , to look for any bad connections or look for voltage and ground point differences with a meter. check engine block to ground that way.

i like to stick my battery posts in a jar of water mixed with lots of baking soda, leave them to bubble away, it does a nice job and also neutralizes any acid. then Il use a bit of sandpaper or steel wool to brighten them up. you can also put some electrolytic grease or battery post protective spray on them to prevent corrosion.

I'll also take a rag and soak it in baking soda and water. let dry then lay that under the battery to neutralize any acid that should migrate, It will prevent the battery box from rusting. you can wash your battery case with that just keep it well away from the top and the cells themselves , you dont want baking soda in the battery but that can save you a pair of jeans if you pick up the battery.

on some 240's they run wires near the front of the oilpan a bad area for oil contamination so they sometimes sort of rot out or have insulation cracks there you can check the condition of wire in such places where there is a lot of oil around.

near the fuel rail there are some important grounds under the bolts that hold down the fuel rail on my 240 and 740's worth cleaning them , a bad connection there can cause a no start.