So when my factory is somewhat idle and not drawing a lot of power, my cluster of reactors all start getting overheated and going into SCRAM constantly. They don't met down, though.

I'm having trouble keeping a stable temperature when not under load.

PS: I thought I could link together cooling towers by lining up the arrows, but all that seems to do is feed cold water into the tower's input, which of course doesn't do anything for cooling the reactor.

Each power plant is a cluster of 4 reactors, all descriptions below are looking at just 1 of them.

The temperatures are set like this:
At 860, start pumping coolant into the heat exchangers. If none of the steam is used, dump it into 1 cooling tower.
At 880, start pumping coolant into the ECCS and the steam cooling tower, continue pumping until it is back down to 860. ECCS is coupled to 4 cooling towers.
At 950, initiate SCRAM and turn on all coolant and steam pumps until scram is complete.

I recently added the bit about engaging the steam cooling tower above 880 - that way it's letting the heat exchangers work even if no power is being pulled from the turbines. This seems to have helped, but it's not enough.

Granted, it seems that in this low load state it can take a while for the heat to build up. Maybe 15-30 minutes. So the cooling is almost adequate. But eventually the cooling is overpowered and SCRAM is activated. I'd say at any given time 1/3rd of all my reactors are in SCRAM when my base is idle. I have 144 reactors running at the moment.

I'm experimenting now with the ECCS cooling towers. I used to have 2, I added 2 more. The towers are dumping the cooled water into a storage tank to feed back through the system. I think the problem I'm having is that the cooling towers can't dump their cold water out fast enough into the storage tank. They get backed up. I think I need more than 2 cooling towers for RCCS, but adding more doesn't seem to make a difference.

I will work on getting a screenshot/diagram together if you think it might help.

PS: I thought you could link together multiple cooling towers to work as a single unit by lining up the input/output arrows. This just seems to feed cold water into the subsequent tower's inputs, which does nothing to help cool the reactor.

Ok I figured it out, I think. Still experimenting but I figured out that the steam cooling tower was getting backed up because it was dumping it's cold water into the primary coolant loop - which during operation is being pumped into by other pumps. So there was nowhere for this other water to go. So I instead made the steam cooling tower's water output go into the main water feeding the plant, so it can more easily get rid of it's water. Seems to be working now!

And by working I mean that it in a low-load sitution, the plant goes up to 880, the ECCS kicks on, it goes down to 860 and starts over again. Previously the ECCS would kick on and the temps would slowly keep rising until it hit 950 and SCRAM.

Glad you are having fun ;)