Mitsubishi Repair & Maintenance Questions? Ask a Mechanic Online
Over heating when driving at upper speeds is nearly always going to be an issue with the coolant itself rather than the cooling of it. Meaning you are still expecting a thermostat or an air pocket in the system, or worse a combustion gas leak that is only occurring at upper speeds.
Any issue with the radiator would be present at low speeds too, and of course any problem with the fan would be not only present at low speeds, but also not an issue at high speed since you have air flow crossing the radiator. The only exception to this would be if there was a substantial blockage externally, preventing air from getting through the radiator.... that would be visually pretty obvious though if it were the case.Since you have already changed the thermostat and we hope gotten all the air out of the system, I would make a block leak test a priority at this point. Most auto parts stores will sell a chemical leak test kit for pretty cheap. The methods vary depending on the type, but essentially the end result is more or less the same... it uses a chemical to check for combustion gases in the coolant. You can also have a shop use an electronic gas detector to check for gases in the coolant too if you have a shop with one available. The other strong consideration for this is that you mentioned that the coolant recovery is bubbling yet you can put your finger in it and it is warm.... if it is bubbling it is only going to be either because it is boiling or because exhaust is entering the coolant, and if it was boiling you definitely wouldn't want your finger in there!
This is where I would make my priorities... check for leaks there through one method or another. With most other items in the cooling system eliminated by either normal low speed operation or by replacement, it is the most likely suspect... and it is even more likely given the bubbling nature you are experiencing.