overheating problem

i have a 1994 honda civic ex, 130,000 miles, 1.6 sohc. after driving it for about 25 min it begins to get hot and about ten min later it will overheat. had a block test done, so replaced head gasket. did pressure test, so replaced upper hose, radiator, thermostat, and did coolant flush. we bled the system, it worked fine for about two weeks then it started overheating again. did not replace these items in any specific order. both fans work (coolant and ac). and no fluid is found leaking. running out of ideas and stuff to replace, can you suggest anything else? it overheats during acceleration and when you come to a stop it goes back down to normal temp. looked at heater core and doesnt seem to be leaking, also checked water pump and it doesnt seem to be leaking. any ideas?? ps temp goes straight up to half way as soon as you start the car.