I was on a trip in Washington state when the overheating started occurring while driving up hills and about 25 mph. When I got to Utah I suspected the thermostat might be sticking. I know, how did I get to Utah without any problems! I use my Mitsubishi as a tow car behind my motor home. I was aware of the jiggle valve and did install the thermostat correctly. I jacked up the front of the car and let the engine run with the radiator cap off till no more bubbles appeared. No contamination in the oil or water is noted.
I don't know it still sounds like it is air bound and by this it may be you could be having the beginning of a bad head gasket.
When this happens compression will slowly leak into the cooling system and form an air pocket in the cylinder head. You can keep bleeding the system but the air still comes back.
You can do one of two things, take out all of the spark plugs and pressurize the cooling system and wait about ten minutes. Then crank the engine over, if the head gasket is bad the coolant will blow out of the bad cylinder.
The other is take the car to a repair shop that has an emission machine and have them put the exhaust sniffer in the neck of the radiator just deep enough to sniff air and not suck in coolant. If the head gasket is bad then the hydrocarbons will show up on the reading where no hydrocarbons should show up at all.
Take a shot we are running out of options and ideas, let me know when you can.