1998 Subaru Overheating!

Hi,
My 1998 Subaru Outback Legacy 2.5l EJ25 has about 126,000 miles on it. One day the car mysteriously began to overheat. Since then I have replaced the radiator, radiator cap, both upper and lower hoses, the thermostat (twice - second time with the Subaru brand), and the temperature gauge's sending unit. The only thing that I have not replaced on the car is the water pump. The car runs fine for about 20 to 30 miles and generally runs fine when I'm up to speed (around 50 to 60 mph). However, when I slow down or am stopping the temperature gauge will rapidly shoot up close to the red zone, and will then rapidly shoot down to normal operating temp when I accelerate. I have had the car tested for head gasket problems as well (hydrocarbon testing, leakage testing, and compression testing - all turned out negative). I'm currently at my wits end at trying to fix the problem and have been trying to avoid a costly repair. Thanks for your response!