I have a 2003 Impreza 2.5 TS. My heater hasn't been working consistantly under the following circumstances: when my car has been parked outside, in the cold for awhile, on a hill (even a moderate one).

I'm finding under those conditions, it will take about 10 miles of driving before my heater kicks in. Even then, after it has begun to heat up, when it idles (at a stoplight for example), it will pretty immediately begin to blow cold air again.

I haven't had this problem at all after it's been parked on level ground.

any thoughts?

Thanks!
-Laura