1) If the tank has black goo inside on the walls, it's possible the head or gasket at some point had leaked and allowed oil into the coolant. It's also possible for such a leak to be temporary and then stop (don't ask how I know). The tank should definitely be replaced as preventive maintenance after 15 years! The new tank is about $50, and is clear plastic (not yellow or brown), and has a silica pack inside to help control corrosion with the aluminum & iron parts in the system. Cheap insurance - trust me. (Photo below.) Monitor the coolant and make sure you're not getting black goo in the system, if so, the gasket is probably failed. Remember that mixed liquids is usualy a gasket, while high cold pressure (and/or erratic high temps) is often a cracked head. Passing a 20psi cooling system pressure test means absolutely nothing, other than the system doesn't have any gaping holes in it. :p
2) The 124/603 (603.960) came with a "blue" 105/128C temp switch that triggers the electric fan at 105C, and kills the AC compressor as a safety at a stupidly high 128C. The 126/603 (603.961) has a "gray" 105/120C switch that costs much less ($25, I think) and I like the lower temps as well. For some unknown reason, the 105C portion of the switch is almost always dead after 10+ years. At least it was on both of my 87's, my sister's, and a co-workers - 4 out of 4, seeing a pattern here? If it's not recent, replace it. Why? Lemme see, $30 for a new switch, $3000 for a new head. Hmmm... tough call.
