The radiator and first head gasket were all replaced before I bought the car. I didn't take ownership until 20K miles after the first head gasket was replaced at 90K. Even still, only 40K between head gaskets does seem like way too soon. The car has never overheated while I've owned it and the previous owner didn't mention anything about it having an overheating "problem", or even if it had ever overheated, even once.
I checked the transmission fluid, and it is clear, so I am sure it is oil that's in the coolant. I appreciate all of your comments and I will, albiet reluctantly, heed all of your advice. I will call my mechanic Monday morning and make arrangements to have it done asap.
Just a few more questions: is this a fairly straight-forward DIY job for a person of moderate ability or should it be done professionally? What is the typical turn-around time for this job for a professional? What should I expect to pay a professional, and what else if anything should I have checked/replaced while the head is off? Thanks again for all of your sound advice.