Bronco overheating

Hello I have a 95 bronco 5.8L which overheats after about 5 or 10 minutes of driving. Also the temp gauge moves up and down constantly where it pretty much redlines then goes back down to read just above cold every couple seconds. Engine gets scolding hot and coolant can be felt boiling in the upper rad hose after about 7 minutes of normal city driving. Water pump hoses fan clutch and thermostat have been changed altho I am not sure if the thermos jiggle valve was facing up when installed. New 50/50 coolant and both radiator and resivoir are filled to the brim. I have refilled the resivoir and squeezed the hoses several times and driven it a couple miles to get air out. Could there still be air in the system causing my problem? Radiator and hoses get super hot which I would think means coolant is flowing properly. I believe rad cap is new. The belt is good and doesnot seem to slip and there is no oil in the coolant or coolant in the exhaust. Could something be wrong with my engine to make it cause excess heat that the cooling system cant handle like low oil? Oil is just below the low mark and is dirty and in need of a change but I cant see that causing a drastic overheating problem