heres the problem i have a 94 integra gsr. when i drive about 70 mph on the free way for about 10-15 mins it starts to over heat. but if i turn the heater on while its over heaying or when it starts to over heat. it stays at its normal temperature or where its suppose to be. i changed the water pump and thermastat the past week but it still does the same thing. No hoses are leaking and theres no coolant leakin from any other place.
can u guys help me out? what can seem to be the problem? thank you.

 

radiator cap

 

imma try it out the radiator cap rite now and ill tell u if it does the same problem. thanks.