What could be the problem with overheating on a 1998 subaru legacy outback symptoms sudden rise in temperature to overheat temperature immediately falls when accelerating block sniffer rules out head?

Sounds like a faulty/sticking thermostat to me. Had same problem with a 99.Fixed it by adding coolant slowly into the small bleeder screw hole ontop of the radiator.Run engine with heater opened to hot.Fill while running, then shut off engine.Repeat as many times as needed till no coolant can be added.Bleed screw till coolant steams out (no air).Any air in system can cause overheating .Very important front of car must be raised 18".You can also add any of the new Super weting agents that keep temperature 20 degrees cooler.Mechanics told me it was a head gasket and gave up.Car running fine one year later.Good luck