One of the earlier replies suggested you check for a cracked head. You mentioned that you had the engine work done years ago. If the head wasn't pressure tested AND magnaflux'd or if you have any doubts at all, then you need to. Everything about what you described is a cracked head. Was the head planed? It may be warped enough the head gasket is leaking.
Some other symptoms. What does the oil look like. Is it milky before or after running it or if after letting it sit when you check the oil plug, does any free water come out? At a minimum you have a head gasket leak, the worst a cracked head.
After letting it sit a couple of days, do one or more of the plugs look rusty or have rust on them?
The water is going somewhere, either in the oil, out the tailpipe, blowing out the overflow (compression gasses).
Some already mentioned by you and others, heating very quickly, heating with the thermostat removed, etc.
Just last week I saw all the symptoms on an old truck I'm helping my son with. I, like you didn't want to think that was it, hoping it wasn't, hoped it was somethng else, and tried to ignore the facts. When I pulled the head it didn't take the machine shop long to tell me what I didn't want to believe.
It might still be possible it is something else, but it sounds like you addressed all the easy ones. This isn't expert advise, but advise from experience (some very recent).
