This is kind of a long story - and a lot of troubleshooting has already been done. Wondering if what I have is multiple issues going on at once...

I have a '91 Civic hb, 217k miles. Weather is in the teens (F) on average.

Recently the engine would only overheat after driving 20 minutes or more at 70 mph+. It did just fine in town, no matter how long I drove. And this is after having the thermostat replaced and an oil change. When the thermostat was replaced the mechanic said that there were no leaks in the cooling system.

Now the car overheats rather quickly after leaving home. The most recent troubleshooting I've done is this.

* After warming up the engine to normal operating temp then driving it around my neighborhood, the heater blows cold air for several minutes, then suddenly blows hot air.
* After a minute or so, it starts blowing cold air and the engine temp starts rising.
* After engine overheats, I found out that the cooling fan isn't on and temp is very high.
* Turning off engine then turning on the ignition the cooling fan will come on for a few minutes then automatically turn itself off (engine wasn't running).
* I could hear the coolant bubbling/boiling in the overflow tank after turning off the engine.
* After the engine cooled down I checked coolant levels, and it was a bit low, so I put a little more into the overflow tank, like my Chiltons said.

I'm a bit stumped based on these symptoms. Any ideas?

 

Fan seems to be working since it does come on. since it's not coming on at the right time, it sounds like the thermo switch on the back of the block might be at fault.
However, it's odd that you are overheating.

Was it overheating before you took it in to have the thermostat changed out?
My guess is that YES it was overheating before.
I would start with having the headgasket checked.

 

Does the heater only work when you're idle then get cold when in motion?
How new is your Radiator cap?
Is your coolant level going low without any signs of a leak?

I'm thinking leaking headgasket. I could be wrong.
But i'm assuming the headgasket is blown between the cylinders, causing coolant to leak into the cylinders. Which then lowers your coolant level, then which would cause your heater to blow cold then hot.

Check your oil level, is the level higher than normal? Does the oil look milky?
You can perform a leak down test to see if it's the headgasket.

 

remove radiator cap, start car, let it run with cap off...look for bubbles etc. if it bubbles over then its your head gasket.

 

This is a good idea. It should also bleed the system if that was your problem. It does sound like a bad head gasket causing a leak. Can you look at the exhaust? I know it's cold, maybe you can smell it to check for coolant in the exhaust.