98 sable wont heat up

i have a 98 sable, it takes forever to heat up. i drive about 15 mile to work and the only time it gets warm is when pulling a hill. i've replaced the thermostat, water pump,flushed system. its better but not right.it gives good heat whenever it does finally warm up.

What is the temperature rating of the thermostat? How cold is the climate?

i put a 195 thermostat in it, tested it in the driveaway with thermometer on thermostat housing it goes to 200 then opens. it took 25 minutes to get up to temp. it was about 28* outside.

Do you have a block heater?