1996 Toyota Celica High Idle and quickly overheating.

I noticed the other day on the way to work, which is literally a mile from my house that my car was overheating, as well as a erratic high idle in 2-2.5k it would fly up and drop down sometimes relax and be normal. Within the last 2 years I've replaced the water pump and radiator, the timing belt is also new. Before this happened I had noticed it spilling out a bit of coolant after getting off the expressway but thought nothing of it because it was summer, now it overheats spills a bit of coolant within such a short distance and I can't figure it out. I was thinking a thermostat and/or a vacuum hose leak but neither of those problems cause both of my symptoms do they?