Flushing the cooling system in your engine seems like a must do to me if there's rusty water coming out of it now. Include the radiator and interior heater core.

On my current 245 a neglected cooling system caused damage to the water channels in the cylinder head at the head to gasket surface. What seemed like a head gasket failure was actually corrosion of the head's water channels. Presumably of standing for long periods of time or running coolant with depleted corrosion inhibitors, or maybe even straight water.
It happened when a PO owned the car, so I have no way of knowing for sure what caused the damage or why.

Not until after the head had been sand blasted became the damage apparent and was it time to source a replacement for the cylinder head.

So, after recommissioning your B230, keep an eye on the coolant level and surface in the reservoir.

I was suggesting that the lack of a temp comp board causes the fluctuations, but after looking at Art's site, I'd imagine it might still be in there but it may be that the flimsy connections between main and temp comp board are contributing to the erratic behaviour. In a car that has been sitting somewhere for that long, it would actually be the first thing I'd check for. Perhaps taking it out and reseating it a few times scrapes away the oxide layer.